Doris is right. If you use 요 with someone, you say 사랑해요. If you don't use it with them, you say 사랑해.

Yes,these two phrases mean have the same mean:I love you. But 사랑해요 shows more respect. And according to what I've learned, between the lovers "사랑해"is preferred.After all the relationships of lovers is sure to be very close and there is no need to use"경어".^ ^