The answer above is right. What I want to instruct is that they have slight difference in NUANCE. First, 사랑한다 is a little literary and strict, so a man(guy) or a father usually use that idiom. 사랑해요 sounds like more kind, soft and smooth so that woman or girl generally use it. (Of course a man can use this one. Honestly I did >.<)

You mean " sal[r]ang'handa 사랑한다 " and " sal[r]ag'haeyo 사랑해요 ".... They're almost in the same meaning :)