Honestly it comes and goes. Sometimes you just have to either slog through the valley, or accept that you're in a valley and seek joy elsewhere until you get through it.
All the while, keep looking for new stuff, staying curious, learning.. that'll insure you have a big backlog of interesting things to get motivated about the next time you're back on an upswing.
it's also just reasonable to get a little depressed sometimes. It's not always obvious in the moment what is causing it - if anything.
At my company we try and remember that we and our colleagues are not bottomless motivation machines. we have highs and lows, and it's best to recognize when you're in a low and adjust your own expectations and those of the people around you.
