Honestly, when I'm happy/ in a good mood. That's when I'm most naturally motivated and full of energy and such, and when going for a run sounds 'fun', almost.
It's why exercise is a bit of a catch-22.. when it may be the most beneficial to you, is when you're probably more in the dumps, thus it's harder to force yourself to do it, even though at that time it would be better because by doing so it would get the endorphins going.
So to get out of the element of having to always personally motivate, I imagine that's where having a defined schedule / actually paying for it (whether having a trainer, or being part of a team/sport, or simply having others to be accountable towards), where you need to show up at a certain time each week or are part of a group exercise program or class, could be more beneficial for some.