I ALWAYS have WAY more seed that I can plant due to many, many factors.
One, of course, is that I am a sucker for anything new or interesting, even if I don't actually have room for it (I think I worked out once that, if I restricted myself to only the amounts I could actually plant in a given year, and got a "normal" survival rate, then one pack of seeds would generally probably last me 10-20 YEARS!)
Another is that I am in constant battle with my environment. Leaving aside the never-ending attacks by critters, my soil is super rocky and super acidic (since most of our trees are oaks and hemlocks) and our sunlight, basically zero (with no legal way to get more). And we just don't have the money or time for the MAJOR overhauls it would take to even BEGIN to correct these problem (or why I laugh bitterly whenever people say "just dig out all your garden space to about 9-10 feet down every year, and fill it up with fresh topsoil"! So getting thing to grow is rare, getting them to grow WELL, almost unheard of. Therefore a lot of my buying is an unending attempt to find SOMETHING that will actually work.
And, of course there is the fact that a lot of my seed is "found" as opposed to ordered from a seed company. And one big catch about "finds" is that they DON'T happen on a set schedule. You have to take things as they come, and that often means being prepared to hold a LOT of seed when a large find comes along, since you'll probably only get one chance to take advantage of the opportunity.
The flipside of this is that I often forget that seed isn't immortal, and end up delaying planting it WAY longer than I should. Probably the apex of this was when I finally got around to planting out the contents of my "tomato cabinet" where I had been hoarding various tomato seeds going back to when I was in COLLEGE! 4-5 POUNDS of tomato seed went in, a grand total of ONE actually germinated and grew. Everything else was long since kaput (including quite a lot of seed I got through unusual sources, and hence had no way of ever replacing.
I SHOULD have a designated seed freezer, but we just don't have room for it in the house (and given how full both of them are, we generally don't have any room for any seeds in the EXISTING freezers either). So I sort of have to just keep them dry and hope for the best.