Hey! Here's a super late, but sincere thank you for faving my work. I'm really happy that you enjoyed it and terribly sorry for such a late reaction...
Don't worry about it! I actually don't understand the need for artists to thank every single one of their watchers? Like, I understand that it's polite and a show of good courtesy, but is it really necessary? That's just me, though. You're welcomed.
For me personally it's a way of showing appreciation to people who enjoy my work. Essentially: a watch/fav makes me happy, so I thank the person that made me happy. Of course it's totally a personal preference.