Dr Love here, telling all you cats and kittens out then to profess your love whenever you feel it in your heart! However, 'I love you' is a powerful phrase, so it needs to be built up to - show you care, give them affectionate acts, playfully tell them how much you mean to them - let your eyes show them how much you love them, and when you've given your actions time to breathe and its clear they are liking them... tell them you think you love them :)