Be as honest as you can if you love her tell her things she would love to hear. I love you because you are yourself around me because I know that you are the only one who can make me smile as much everyday like you do. I love the beautiful person you are and life without you would just seem so empty. If you don't love her, don't respond. It would hurt too much.
Obviously you tell her why you love her, if you can't answer this question then do you really love her? I know guys aren't into their feelings and stuff but all you have to say is something like, I love you because you are the best thing that's ever happened o me. Words that should come naturally when you love someone.
The answer should come easily. When I ask my husband he says "Cause you are fun, silly, caring and intelligent. " He doesn't have to think about it. When he asks me I tell him because he is strong, smart, caring and a wonderful person. "
If you have to think about it, then I have to you need to evaluate if you really love her.
i feel really bad, once my boyfriend asked me why do you love me ? and I said I don't know. this was about 1month ago (we've been going out 2months) and I didn't mean it as in I don't know why I love you I meant it as in there was so much stuff to say I didn't know what to choose.this caused a huge argument and he took it as in I didn't love him
so never EVER say you don't know just tell her the truth, whatever you say she will be happy about