If there's someone you click with and desire who seems "cooler" than your SO and you really really want to be with them but you're monogamous, the only way to ever be with them would be to leave, to "jump ship". You also might find yourself romanticizing someone you can't have, since you never have to experience their bad with their good. Whereas if you're poly, you get to fulfill your desire to have a relationship with this cool person, and get to see them in a realistic way, without leaving your SO. In that light, it seems to me like poly is a way to *preserve* existing relationships in the face of bright-and-shiny, not destroy them, yeah?
That said, determine what you need to maintain passion, connection, and mutual interest with your SO -- regular date nights, a new shared hobby, a commitment to trying a position a week from the kama sutra, couples counseling, whatever -- and do that without fail, whether or not you start seeing anyone new.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.