What most often makes you go sit down and tie some flies? Is it that you need to fill your gaps in your fly box (lost, expired, etc) or is it a new pattern that you happen to find on the internet?
Mostly when I need to refill my boxes or going on a trip.
When I get low on stock or want to create something new.
It depends. When I get time I usually just tie what needs to fill up the box. Either that or I tie something that looks interesting.