I don't know, I personally don't find it very fun to lose unless I am more concerned that the other players enjoy the game enough to want to replay it. Generally, it becomes hard to convince IRL friends and family to play Dominion or similar games with me because they don't like to lose to me, so maybe I am not alone here. Online, I assume my opponent is playing because they want to, and so I don't have much incentive to hold back.
What drove me to get better was this sense of "never again". "Never again" will I lose because my opponent mirrored my engine but got Horn of Plenty when I didn't, "never again" will I lose because I stupidly ignored Scrying Pool, and so on. In this vein, losing to higher level players in a demoralizing seems like a possible catalyst for getting better. I fully acknowledge this as being negative reinforcement, but I cannot deny that it played a major role in me getting better at Dominion.
Yeah, there are some cards and strategies I am more fond of than others, but I've made efforts to detach my preferences from my actual decisions. Fortunately, I do find satisfaction in the act of finding the path to victory on a given Dominion board, especially since path can look very different depending on the board (engine, slog, rush, "good stuff", etc.).