I am going to spout something I learn from my psych class (I minor in it when I got my BS and hopefully going to get my master someday and ph.d in it)
Some got into it cause it feels right at the time. People in general are a "group" type of animal. You don't see many loners out there. Human race NEEDS interaction on a regular basis. It what help us grow and hopefully make us better people.
personal opinion
Why settle down? cause people hate to be alone. As a person gets older, he/she starts to think about being alone in the future. There is this thing call "circle of influence" (CoI) that shrinks as you get older. What is CoI? Basically when we were younger, we have a larger pool of people to interact with "new blood" so to speak. When you go from elementary, Jr. high, High school, and College, your CoI increases. Of course some social group keep your CoI stable but it all depends on YOUR interaction. When you were in college (or in college) you usually can meet up random people without trying too hard. You meet them in your class, you meet them at lunch/breaks, you meet them in social outing. This increase your CoI. Your CoI shrinks when you get a job after college. Your people interaction are limited to your co-workers. This can be a good and bad thing (depending where you work) of course then it rely on YOU to really go out and make friends instead of "randomly" meet people. some is easy for them, for some it is hard as heck.
to me, CoI usually work in-person type but also can work on the Internet. It depends how much interaction you have with those people. This forum is a good place to "sustain" friends in your CoI. Why am I talking about this/
It brings down to your "chance" to find a mate really. (my term mate means life partner. It doesn't have to be opposite sex) A person you feel comfortable with, want to spend your life with, and sometimes is the most important to you (making that person happy) as your CoI shrinks, your chance to meet your partner also shrinks. sure there are services and such that "gear" toward that, but it is a toss up really.
Love... I think a lot of people have some "misconception" of what love really is. some of us like the idea from the "movie love" but it is more than that. It is being there for someone and they are there for you in return. Your life partner usually will be with you regardless of anything going on with your life. You can share your intimate details with him/her and be comfortable about it.
I guess this is kinda my long wind version of why people "seek" to settle down early or late in life. Of course other circumstance may bring two people together.
Knocked up.
"just click"
financial dependency
stability
there are tons of other stuff that could cause people to "hook up"
(I hope I made some sense
I try to type this on the fly at work)