Actually, they don’t. If you think about it, a child spends from birth until about four years of age doing nothing but language learning—for eight or more hours a day. That’s quite a bit of time on task. It seems effortless to us because the child seems to do it without thinking about it. Children engage in play, eating, and just “existing” while simultaneously learning language. They really have nothing else to do with their lives. Adults, on the other hand, already have one if not more languages—as well as preoccupations other than eating and playing. Adults also like to have control over their environment as well as how they learn. And therein lies the tension. Language acquisition is not something you can force or control as you might with other kinds of learning; as with children, it’s something that happens to you over time.
So, what is the optimal situation for language acquisition to happen to you? A combination of lots of exposure, a sympathetic environment, and motivation + patience. Remember: a child learning language takes four years, eight hours a day to get to a system that resembles what will be his or her native language. You’ll need a “stick with it attitude” and lots of good interaction with Spanish; then you can sit back and be one of the millions of multilingual speakers around the world.