I think Rumple fell in love with Belle the moment he saw her...how strong she was, how decisive, how brave...and he wanted that kiss but had no idea it would break his "curse." Their relationship was so tragic because he felt he wasn't enough without the Dark One's dagger and she knew he was more than enough without it. I don't know if you saw the ending of the series but, for these two, it was a perfect and beautiful way to begin the next chapter for them.