It's the satisfaction of looking at an object that was created by my own hands. I'm a woodworker by trade and it's the same driving force. Getting paid for my cabinet work is secondary to viewing the finished project. It's also comforting to know that hopefully what we build will outlast us and bring someone else some happiness or sense of pride of ownership. Although the internet has brought us together, it's also a nice time of solitude to work on a model, just getting lost in thought, clearing your head of other worries and concerns. The same applies to my woodwork. I tell people it's my happy place.