What science won't say because of the subjectivity, which works for both human and animal is that a tilt of the head is an open emotional signal for contemplation, and can mean the subject is comfortable and even enjoying what they're looking at. It's seen as cute because often the subject itself is finding something to be cute, adorable. It can really send a strong but sophisticated sexual or affectual signal to look at someone like that, while the eyes will be the distinguishing factor between those two meanings.