Was thinking about this today a fair bit, there are four key motivations to take on a project:

The Brief - is it so compelling that I would regret not doing it.

The Learnings - is it allowing me to gain new skills or expertise.

The pay - is it paid to good to say no.

The team - is the team just a joy to work with.

One thing that dawned on me is that I realized the oldeer experienced I get the more the weights are shifting. While pay, brief, and learnings need to be balanced, nothing matters more than just who you work with.