Likewise, adding a partner to the mix is not likely to "spice up" your relationship if someone isn't getting their needs met. People are not need-filling machines. It takes a lot of communication, self-reflection, and emotional maturity to maintain romantic and sexual relationships with multiple partners.
We don't always choose metamours
In polyamory, the person your partner is dating besides you is referred to as a "metamour," or the love of your love. It's really a wonderful situation when everyone can hang out and play Cards Against Humanity together. You may not be attracted to your partner's metamour, but accepting him or her as your partner's partner and maintaining a cordial -- if not friendly -- relationship makes everything a lot less sticky.
I love being friendly with metamours, but there have been a couple of times in my experience when I had to ask myself, "How can someone I love, love someone like her? We’re so different!" Part of the joy of polyamory is, for some people, variety. That means you might always like the person that your partner dates. But it takes a lot of stamina and emotional maturity to smile and be polite with someone that you don't have friendly feelings toward.