A couple of factors come into play for me.
First off what's working which takes some guessing and or hatch matching when appropriate. Once I dial it in, I don't care what it is as long as the fish don't.
The second is the most important, that's the "fun" factor. There are certain flies, methods, techniques I love in fishing and others I loath.
For example, I fish conventionally and LOVE fishing soft plastic baits for bass on a casting set-up, but I would quit fishing versus working a plug or spinner bait all day long on a spinning rod.
The same thing goes with fly fishing, I really don't enjoy fishing streamers despite some of the fish I dredge up so I avoid it as long as I can. I also loath big fly/big water/big rod/sinking line fly fishing so I really only target trout, panfish & smallmouth with a fly rod also incorporating conventional tackle for the warmwater species.
Additionally, I hate fishing fussy dry flies that are delicate or poor floaters that need to be constantly dried, straightened out and redressed after a fish or a handful of floats. Oh, I'll use them for the Mensa trout when necessary, but I quickly switch to something else for the rest of the morons. However, I can fish a foam beetle all day even if it not the best choice because they float like a cork without floatant and I love just fishing and not fussing.
The same thing goes for searching the water with a nymph or soft hackle. I love not worrying about how a fly is floating, is it floating, will it still float after a fish while I am prospecting potential pockets of piscatorial pleasure.
For me, the act of fishing has to be fun regardless of results and everything after that is a bonus.