There are two gemarot which discuss the recitation of the berakha of shehechiyanu upon experiencing unique joy or simcha. The mishna in Berakhot (54a) describes someone who purchases a new home or new clothing, mandating the recitation of a shehechiyanu. An ensuing gemara in Berakhot (58b) describes a berakha of shehechiyanu upon meeting a friend whom one hasnt seen for a period of 30 days. Presumably, the common denominator of these two shehechiyanus is the joy which the respective experience generates. Purchasing a new home or reuniting with a lost friend each create joy, which in turn is the source of the chiyuv to recite shehechiyanu to thank Hashem for sustaining us to this moment, which has caused such immense simcha.
A different type of shehechiyanu may emerge from the gemara in Sukka (46), which describes the berakha upon the first performance of a Yom Tov-related mitzva, such as lulav or sukka. The gemara debates the appropriate moment to recite the berakha upon first interaction with the mitzva of the chag (preparing the lulav or sukka) or during Kiddush announcing the commencement of the chag. Either way, it seems that the CHAG and its mitzvot generate an obligation to recite shehechiyanu.
The simple approach would be to group this shehechiyanu together with the first paradigm. Experiences which generate simcha obligate a berakha of shehechiyanu; a chag and its mitzvot would certainly fall under that category. In fact, Tosafot in Sukka (46a) question the shehechiyanu recited upon chag-related mitzvot (sukka, reading megilla [Megilla 21]) in light of the fact that the performance of a brit mila does not warrant shehechiyanu recital. To further complicate matters, Tosfot quote a gemara in Pesachim which demands shehechiyanu for pidyon ha-ben, inferring by way of omission that mila DOES NOT require shehechiyanu.
Tosafot solve this riddle by claiming that any mitzvah which generates SIMCHA mandates shehechiyanu, which explains the requirement for Yom Tov mitzvot and pidyon ha-ben. Mila should have been a candidate, but since the baby suffers, the SIMCHA is slightly compromised and the shehechiyanu thus omitted. It is clear that Tosafot view the shehechiyanu of chag and chag-related mitzvot as based on the JOY that the chag causes. Pidyon ha-ben also causes joy and deserves the berakha, while mila does not allow for the same unmitigated joy and therefore does not merit a berakha.
The first hint that a different type of shehechiyanu may exist stems from a gemara in Eiruvin (40b), which questions the recital of shehechiyanu on Rosh Hashanah and Yom Kippur. On the one hand, these days are not identified as days of simcha (itself an interesting point), and therefore MAY NOT warrant shehechiyanu. Alternatively, the gemara asserts, since these days occur only once in a while, perhaps they do deserve shehechiyanu. It seems as if the gemara is building a new paradigm for this berakha: not only should the berakha of shehechiyanu be recited during moments of SIMCHA, it should also be offered as we pass important milestones that dot our yearly schedule. Perhaps important milestones - even if they are not experiences of joy per se - should mandate a berakha praising Hashem for sustaining us to this stage. Reciting shehechiyanu on Rosh Hashanah and Yom Kippur would be based solely upon recognizing a new template for this berakha. Not only should the berakha be recited for simcha generating experiences; it should also mark important halakhic milestones.
In fact, the gemara may provide an additional application of this second form of shehechiyanu. Commenting on the question of reciting shehechiyanu for Rosh Hashanah, R. Yehuda remarks that he would regularly recite a shehechiyanu on a new fruit. He implies that just as he would recite the berakha for a new fruit, shehechiyanu should be recited on Rosh Hashanah and Yom Kippur. Presumably, R. Yehudas point was that this ALTERNATE model of shehechiyanu exists, indicated by his minhag to recite the berakha over new fruit. Eating a new fruit does not generate unique excitement. If a shehechiyanu is nonetheless recited, evidently shehechiyanu can be recited in response to annual milestones (ripening of new fruit). As such, it can also be recited for Rosh Hashanah and Yom Kippur - different types of milestones. R. Yehuda assumed that shehechiyanu upon new fruit proves the existence of a second model of shehechiyanu which can be applied to Rosh Hashana and Yom Kippur even though no simcha exists on those days.
It is unclear whether the gemara would accept R. Yehudas premise. Perhaps the shehechiyanu upon eating new fruit is comparable to the shehechiyanu upon moments of simcha, such as chag related mitzvot or purchasing a new home. It is possible that only ONE model of shehechiyanu exists, encompassing obvious moments of simcha as well as a less obvious ones, such as eating new fruit.
There are several particulars about the halakha of shehechiyanu upon new fruit that may help determine whether the berakha is recited upon the joy that the new fruit causes or in response to the milestone that the seasonal ripening of fruit represents. Perhaps the most reflective issue is WHEN the shehechiyanu upon a new fruit should be recited. The simple reading of the gemara implies that one should recite the berakha upon his first encounter with the fruit even if he does not eat it. The Rambam cites this halakha in Hilkhot Berakhot 10:2 and writes that a person who SEES the new fruit should recite the berakha, confirming our initial impressions. A similar perspective emerges from Rashis comments to the gemara in Eiruvin that recorded R. Yehudas minhag. The Rosh however, after citing these positions, cites an accepted minhag to only recite shehechiyanu UPON EATING THE FRUIT; he also cites several Ashkenazic Rishonim (among them Tosafot and the Maharam Mi-Rotenburg) who maintained this as halakha.
Perhaps this question stems from our initial one. If shehechiyanu upon new fruits is based upon the fact that it is a milestone, independent of any joy or simcha, it should be recited when a person sees the fruit growing. The seasonal ripening of fruit indicates the reaching of this milestone, and the berakha should be recited. If, however, the berakha is caused by the joy which the new fruit causes, presumably that intense joy is only experienced when eating, and the berakha should be delayed until that point. There are some Rishonim who justify the position of reciting the berakha upon first encounter by claiming that a person experiences simcha even when seeing a new fruit (see Semak 151). However, the more logical approach would be to explain this position as viewing shehechiyanu based on milestones and not simcha.
In theory, the inverse situation may also serve as an interesting nafka mina. What would happen if a person eats a new fruit that he does not enjoy (think the second night of Rosh Hashanah!). Certainly, the milestone of ripening fruit warrants the shehechiyanu, but one may question whether this new fruit actually causes simcha. The minhag, of course, is to recite the berakha even if a person does not enjoy the new fruit.
Another debate may concern reciting a second shehechiyanu upon a processed fruit. For example, if someone recited a shehechiyanu upon grapes and subsequently drank grape juice or wine, should a second shehechiyanu be recited? The Terumat Ha-Deshen (33) rules that a second berakha should be recited, while the Agur quotes Rishonim who are less certain (see Tur, Orach Chayyim 225). Perhaps this technical question is driven by the original fundamental one. If the shehechiyanu marks the milestone of grapes ripening, only one berakha should be recited. However, if the berakha marks the unique pleasure and joy derived from the new fruit, in theory, the new pleasure of grape juice should warrant a distinct and second berakha independent of the original berakha recited upon the pleasure derived from grapes.
This question may lead to some interesting nafka