I've always found that it's very rewarding and fun to create and build my own designs, and usually
see them work as intended. I like problem solving (my wife might question that while
I'm doing it) and also helping folks out when I can.
Given those motivations, open source is sort of icing on the cake for me. But that is not to minimize its importance, because it's a big enabler for a lot of things including information sharing and community-building like this Arduino forum. There are a lot of impressive folks here that really know their stuff and are willing to share their knowledge and assist others with their projects. Indeed, open source has fostered an entire ecosystem that runs from hobbyists to small businesses, to distributors and service providers. As testimony to the importance of the open source movement, many large companies, including semiconductor manufacturers, have taken notice and become actively involved.