Why does this hert me ?

i have a crush on some one allot older than me and he is leaving the school soon and iv had a crush on him for about 2 years and he is like 15 years older than me and ever time i think abot him i smile and get butterflys but then if i think him leaving it makes me really sad and my tummy herts :P .. and i feel like criying and so on ... i know i can never date him but why does this hert so much would it be because i love some one i cant get and i cant admit i cant have him ? please it herts so much thinking about him leaving x