That's absurd. No matter how much I love what I do, spending more than a certain amount of time at it makes me tired. Tired me makes mistakes that fresh me has to fix. Tired me produces less overall output for the same amount of effort put in. If what I want is to maximise output, stopping to rest is the only rational choice.

Also, the implicit assumption is that, if you love what you do, you don't love doing anything else. I want time to devote to activities other than the work I so love. Much of that time can and does end up producing useful cross-pollination with my day job, but that's just gravy.