You are only 14. If your friends are unwilling to tell you why they aren’t crazy about your choice, then it could be something as simple as another girl is also crushing on him, or they heard some rumor. At your age there is a lot of quick judgement and drama. Be true to yourself, be honest, don’t cheat, and take mental notes as to what you do and do not want in relationships. Above all else, respect and be respected. Don’t accept less than being treated respectfully. Set that bar high, early. With your friends and your crushes.
My guess is the reason is minor. Hopefully they would tell you if he was doing drugs or killing cats or something.