I was blessed to have a professor who really stressed to his undergraduate students the importance of the historical and the religious of the three pilgrimage festivals (a.k.a. Shlosha Regalim): Passover, Shavuot, and Sukkot.
The funny thing is, they've all but lost their agricultural (that is, pilgrimage) meanings, and they've come to mean a variety of things: Pesach being the holiday where we eat matzo and do those annoying seder things; Shavuot being that holiday where we stay up all night studying Torah and noshing dairy; and Sukkot the chag where we sit outside in booths and swat at flies. But what do these chagim really mean, and why are they tied together so tightly? In reality, you can't have one without the other, and if you celebrate one or two but not the others, you're really missing the point.
In a nutshell, agriculturally and historically, Passover starts the grain harvest, Shavuot marks the end of the grain harvest and the beginning of the fruit harvest, and Sukkot marks ... you guessed it ... the end of the fruit harvest. Living in the Diaspora, you really miss the sense of the seasons, and as such you really don't get these simple and basic histories behind the holidays.
Of course, each of them have religious significance as well, with Passover marking the Exodus, Shavuot marking the giving of the Torah at Sinai, and Sukkot commemorating the living of booths in the Wilderness of the Exile. Again, these historic/religious events are cyclical and play a part in a timeline that is, without a doubt, connected. You can't remember the Exile without remembering the Exodus, and the giving of the Torah is sort of insignificant unless you understand why it was given, where it was given, and how.
What's my point? My point here is that these holidays aren't just about our modern observances. Much of what we know about our modern observances (especially about Sukkot) come from sort of a mishmosh of understandings of the Biblical and Rabbinic texts, and although they are just as valid as everything else, it's the basics that are found in the Torah -- in regards to the agricultural festivals -- that really evolved these three pilgrimage/agricultural festivals!
Are you still jonesing to know why we down lots of dairy (and Lactaid) on Shavuot? There are a few interesting and compelling reasons for this. Perhaps my favorite being that the Israelites didn't know how to properly take on meat before the giving of the Torah, so they opted for dairy, dairy, and more dairy, until Moshe came back down and told them how to properly handle meat. Another popular opinion is that it is the sense of Israel as the "land of milk and honey" that appears throughout the Torah that is cause for us to get all milchig over the two-day chag in the Diaspora.