Because you're immature.
Infants and children are selfish because they cannot think beyond what (they) want or need. They lack compassion and empathy for others. It's all about (me). Generally speaking as we get older we understand the world wasn't just made for (me) and what I can take from it. When you're "in love" you won't be as selfish. To love others is to put their needs on level with yours.