If you knew someone in a non romantic way who thought of you as a friend and who cared about you as a person and thought you were special through words and acts of thoughtfulness and one day they said I love you... How would you feel?

I would probably assume that this person saying "I love you" was meant for a platonic type of love rather than a romantic love with the given background. 

So if I were in that situation, I'd probably not think too much about it and tell them that I love them too and that they meant a lot too me.