Being physically with someone is entirely different than being emotionally invested in your partner. It takes time to be emotionally involved and to actually love the other person. When you develop deeper feelings and a bond that goes deeper than superficial things like physical appearance and qualification, then you want to share your feelings with the significant other. When you get to a stage where you want to say "I love you", it's like hitting a milestone that leads you to deeper intimacy. But here's the catch: you need to be sure when to say I love you without worrying about your partner's reaction. You have to say it when you really mean it and when you feel like feelings could be mutual.