When my boyfriend and I first started dating a couple of years ago, he already had four good sized tattoos (calf, chest, and two the same upper arm) and I only had one relatively small one. We went to a convention with a friend that is pretty covered and my bf got a small one somewhat spur of the moment.
I've always loved tattoos and loved that he had some, but it was crazy to me that he would add another without thinking/planning for a while. But now, as I plan future pieces, I totally understand the itch. He's super supportive of me getting more, and even a little bit jealous that I'm next :)