Ezra Klein interviews the person who came up with the individual mandate:

Tell me about your involvement in the development of the individual mandate.
I was involved in developing a plan for the George H.W. Bush administration. I wasn't a member of the administration, but part of a team of academics who believe the administration needed good proposals to look at. We did it because we were concerned about the specter of single payer insurance, which isn't market-oriented, and we didn't think was a good idea. One feature was the individual mandate. The purpose of it was to round up the stragglers who wouldn’t be brought in by subsidies. We weren’t focused on bringing in high risks, which is what they're focused on now. We published the plan in Health Affairs in 1991. The Heritage Foundation was working on something similar at the time.

Yadda yadda yadda. Short answer: He's a socialist who disdains the Constitution and set out to strange the last vestiges of freedom in America.