I am in love with the person I am turning into.

Couples in the letter-writing era fell in love with someone assembled from paper — a voice, a few habits of mind, a great deal of imagination. Then a train arrived, a real person got off it, and the work of loving them began properly.

Turning into is that arrangement pointed forward. Somebody is coming who you can only partly picture, and who is being shaped by decisions you're making this week without ceremony.

Being fond of that person in advance is a strange and practical thing to do. People tend to treat well the person they expect to like.