My background:
I'm a native Midwestern American English speaker.
I started learning Spanish in middle school because it was required, but I think we also had a bit of Spanish in my kindergarten (I can't remember for certain). My middle school class was really boring because the teacher thought she could teach us by immersion when we only had class for 25 minutes per week and were mostly absolute beginners. I think I only learned four or five words in that class. Luckily I've always thought languages were interesting, so when I had a chance to start some high school classes a year early, I took the opportunity to take Spanish at the high school instead of at the middle school. So I ended up having 5 years of high school Spanish.
My sophomore year of high school I had some extra room in my class schedule so I added German. Our German class was really slow though--we only did two years' worth of coursework in three years. I took another semester of German at college, and minored in Spanish. I studied in Cuernavaca for three weeks in an intensive summer abroad course, and then got a license to teach Spanish (which I've never used... oh well).
Throughout high school and college I had several friends who were international students. I had friends from Norway, Finland, Korea, Taiwan, Jamaica, Ghana, Tanzania, Brazil, and more. Usually I was able to convince them to teach me bits of their languages. Also in high school I learned Tengwar (the writing system for Tolkien's Quenya and Sindarin), and in college I learned Aurek-Besh (one of the writing systems in Star Wars).
I learned Korean while I was teaching English there, and I'm learning Japanese now while living in Japan.
My Finnish friend in high school taught me more than some of my other international friends, and I always had a desire to pick it up again someday. And then I found aRtD and SSSS, and decided to continue learning Finnish, and add Swedish to the mix.
I'm also working on making my own conlang(s), but it is slow going.
Unfortunately I tend to forget old languages as I learn new ones, so don't expect me to be able to hold a conversation in any of the above languages, at least not without a ton of mistakes.
Some of my reasons:
I sometimes think of languages like secret codes. I like to be able to keep secrets, and I don't like people keeping secrets from me.
Also I love to travel, and my dream job is to travel the world as an English teacher while learning the local languages. I already have my dream job.
Another reason is that learning languages helps me understand different points of view better, because in some ways the languages in which people think also influence some of the thoughts they have. And it also helps me understand why I think the way I do, and how my own language works.