Various reasons--mostly, a shift in my research interests and needing/wanting to show a commitment to this new area. The second project sprung directly out of my clinical work, so...
Initially, some of the faculty were quite opposed to the idea of me doing multiple theses and some still have their reservations, but I've worked with them to try to address their concerns---I've done some workload lightening in other areas to compensate for the increased workload in this one, and winning our Undergraduate Research Conference helped as well, I think.
I don't really think your drive or tackling of multiple (presumably ambitious) theses is average for an undergraduate. I mean that as a compliment! It just sounds like you're going a bit above and beyond what is required. An undergrad thesis should be "original" in that it is your own work and is an interesting take, but it doesn't necessarily need to have a great impact in the field that would be the hallmark of a truly novel approach or creation of theory-work. Juillet makes a great point: it's difficult for an undergrad to create a thesis in an area where there is little previous work. It's designed to be smaller and less expansive, so to OP, I would say not to stress the originality of your argument, but come up with one interesting question that is a small step beyond the current literature but still within your abilities as a scholar. I keep thinking of the old joke... Q: what is the best kind of thesis? A: a done one.
