Well I am a cheater & I have many reasons. Some r probably strange to most people.
If your philosophy is that we as humans are descended from mammals I wonder why only about 12% of mammals are monogamous.
So how did humans become monogamous? That varies. Some believe it came with the adaptation of religion & taking a female as a possession & according to religion it would be wrong to take something that's not yours. So we're these rules human made or from some type of God.
Really depends on what you believe & how far down you scratch the surface.
Do you search for the Oprah answers or spiritual answers etc etc.
You asked the question of why people cheat & I think you'll get many different answers. There is no universal answer.
And if a person cheats does that mean they don't love you....are you asking other monogamous people, polyamorous people, people that share your same ethnic beliefs?

Where do you stand in your core values & does your boyfriend share these core values. Have you discussed this with him.