I seem to tire of gaming the most when I am winning a lot. I know, seems counter-intuitive, BUT--if I am mopping the floor with players then I'm not getting challenged. Feel the same way when I get schooled (you know: the one sided fight where you have killed 2 of the oppositions troops the whole game and you are down to a warcaster and a half-dead jack. See #DiceHateMe.) The times I get the most out of the game and enjoy the most are the cliff-hanger games--one dice role could win or lose the whole match!
As for the OP, you could be suffering from "Johnny-osis"--a rare but treatable disease contracted by magic players that love to build decks but aren't as enthused about playing them. The halmark of this disease is a focus on theory, where playing the deck (or army) isn't as interesting as R&Ding the tactics, combo's and other synergies of you force (or deck) and assembling them. In laymen's terms--you already know what it is going to do and how it is going to work and what your respective chances are in any given match, dice willing, because you already spent massive man-hours building it, theorizing it, and even running chart assisted "odds" calculations against the prominent meta in the current tournament scene.
The only cure I know of to "Johnny-osis" in Magic is sealed deck--no planning; just playing and adaptation. The best equivilant I can think of in Warmachine is getting back to basics and playing games on a small scale--mangled metal. Take yourself out of the planning and theory for awhile and give control of the list to someone else--or toss out the list entirely and go boxed set.