Dating one person can be tough, try to add a few more and it can be exponentially more difficult! So why do people do it?
Most of modern society has built relationships around the concept of monogamy, partnering by twos to form long term sustainable partnerships (There are certainly historical exceptions but that would be a much longer post). Whether marriage or long term dating is the outcome, partnering with one person is the standard, socially normalized pattern. This is ideal for many, and can lead to a long term fulfilling relationship built on trust and mutual respect.
Some individuals however come to understand that dating or partnering with one person is not the best fit for them. Some of the reason's people choose non-monogamy include:
1) Creating different types of relationships that would not have been manageable or supportive otherwise, allowing for creativity in making the structure that works for you
2) Creating a stable and nurturing "family" and community to raise children beyond just one partner
3) Healing past relationship wounds by facing new challenges
4) Sexual needs which go beyond the scope or capacity of one partner, desire differences or seeking erotic variety
5) Seeking an alternative to religious or moral structures that limit relational/ emotional/ expression
6) Expanding the love and connection beyond the limits of one partner, or the physical limits that restrict friendships
7) Valuing the love and history with a partner, even when you're connection shifts, by changing the relationship structure and shifting agreements to meet both needs
8) Supporting different sexual identities and lifestyle expressions (ie: bisexual having partners of various genders or a kinky person having a vanilla and a kinky partner)
9) Providing more freedom and flexibility, less pressure in terms of being your partners "everything"
Regardless of the reasons, the transition to non-monogamy is really difficult if you're currently in a long term monogamous relationship. Get a lot of support and see a therapist who can best faciliate this transition. Being non-monogamous does not preclude you from struggling with jealousy, fear, sadness, loss, isolation and loneliness. It's just one more approach to consider, while continuing to deal with your emotions and needs in healthy ways. I'm happy to be your resource if you're in Oregon!
Here's a great resource with some information on non-monogamous configurations and terms!