Why can my boyfriend eat much spicier food than me?

Tolerance for spicy food is acquired through practice. The reason he can tolerate hot stuff more than you is that he regularly eats spicy stuff. The brain gradually learns that spicy stuff isn't actual fire, so it stops reacting as strong to those kinds of pain signals, and can even release endorphins – the chemicals that are also released when you're falling in love and when you're experiencing pleasure. If you want to build up a tolerance, you need to frequently be eating something that's near the top of your pain threshold, and keep upping it. After a few months, you'll be able to handle stuff you could only dream about before, and you'll actually come to love the flavors that frequently accompany the higher heat levels. You can start with something like sriracha, graduate to tabasco or cholula, then visit a hot sauce shop like Hot Licks and spend some time sampling a variety of the stuff they carry (I mention that place by name because they DO let you try samples of almost everything they carry).