[k00b]

For me it's learning but mostly technical learning. This isn't the same for everyone though. It took me a long time to stumble onto this.

Think about finding what motivates you like you think about finding the right diet: experiment and iterate.

[k00b]

Also, Dr. Huberman of http://hubermanlab.com/ has been making the round on podcasts talking about the neuroscience of motivation.

tl;dr We are motivated/rewarded (at the neurotransmitter level) when we feel that we're on the right path. What makes you feel like you're on the right path?

This feeling of being on the right path causes dopamine to be released which quells the anxiety/quit sensation (adrenaline/norepinephrine) that's caused by struggle and is what leads to burnout.