If your goal is to build someone up in love, to understand them more, to spend yourself for their good, then sincerity seems to be a better mode of speaking to do that than sarcasm. Communicate your desire to help, listen, love, value plainly and straightforwardly.
Isn't sarcasm what we turn to when we have given up on a person, or when we are frustrated and don't want to help them or love them? Sarcasm seems to be a symptom of cynicism. It gets in the way of dialogue by stirring up anger and also many times introducing confusion, as it is a less clear way of communicating.
Sarcasm is undoubtedly entertaining sometimes, and furthermore can be a way to present something in a striking way that will break though old thoughts, yet we must be careful to use it in love, not as a weapon.