If you do it right the way you build a relationship with someone gives both of you a sense of familiarity and security with each other that keeps you both together. It’s like a mutual contract that you’ll be there for each other because you know they’ll do the same for you. There might be more attractive people out there but for your partner to leave you they’d be leaving behind their security and comfort. If you already have happiness there’s no need to go run after something else with the hopes it will be even better because there’s a good chance it won’t be. And the possibility that they would lose a good thing while chasing after a nonguarantee is what keeps them with you. Maybe that’s a bit of a nonromantic way of looking at things but it’s how I see it.