The same reasons why girls cheat on their boyfriends.
In fact many would argue boys and girls are too immature to be forming deep loving committed relationships. There youth is best spent discovering who they are, focusing on school, and dating for FUN as a social activity.
Although no teenager wants to hear this but the truth of the matter is they're being "unrealistic" when they believe they've found their "soul-mate" at age 16, 17, 18, or 19. In fact most people haven't figured out who they are!
I can't tell you how many "high school sweethearts" go off to different colleges with the belief that they'll maintain a long distance relationship for the next (4-6 years) and ignore all opportunities to socialize and date fellow students who are actually on their campus. Very few last beyond 1 year.
I know this advice I'm giving you is likely to fall upon deaf ears. After all how can good advice compete with the ignorance of youth and raging hormones?
When it comes to love and relationships most of us (fail our way) to success. If this weren't true we'd all be married to our high school sweethearts!
The vast majority of broken hearts we experience during our youth came from having "unrealistic expectations" and being naïve about life in general. Teens thrive off "impulsive" connections.
Your top priorities as a (girl or boy) is to complete your education, establish a career path, travel/explore and evolve as you figure out who (you) are. Once you have that down it will be easier for you to choose the "right mate" for yourself. There's no rush!
In the mean time date for fun and not to find your husband/wife.