What do you guys think?

My take is that coming to love X is a process whereby I extend my sense of myself to include X.

So when you say:

The answer to the question of, "why does this matter?" is always, "because it makes me happy". So then, the idea of love bothers me, because you sort of throw rational thinking out the window, stop asking why something actually matters, and just decide that this significant other intrinsically matters to you.

...you basically lose me. You're talking as though "me" refers to some kind of unchanging primitive whose boundaries remain perpetually fixed. That's not consistent with my experience at all.

You say you have a very reductionist viewpoint on everything. I wouldn't say that about myself; there are things I'm content to engage with as "black boxes" at a variety of different levels of abstraction, and I don't feel the need to constantly "drill down" into them. But in this particular case, I'm aware that my sense of myself is a not a black box; it's a psychological construct, and I'm typically aware of some of the things that contribute to and constrain it.

Some of those things include aspects of other people whom I love.