This page will help you to find out, to put your feelings in place and discern emotional from rational.
Of course, desiring a mongamous relationship doesn't need justification, but neither does a desire for any other type of relationship (including not being in a relationship at all, as Bella De Paulo emphasizes on her Living Single blog).
But it seems hard to defend a essentially monogamous nature to love itself without first assuming that lovers want monogamy, which is circular reasoning.
One could argue that by its very nature, loving somebody includes promising your affection exclusively, so the other person can reciprocate with confidence.
But this assumes that both persons desire monogamy, which begs the question; naturally, monogamy-oriented people will desire monogamous relationships, but this doesn't explain the desire for monogamy itself!
But this assumes that whatever you give when you love someone is limited or scarce, so that giving some (or more) to Jane or Joe means giving less to Janet or John.
This may be true with some resources like time or money, but not as obviously true with respect to affection; after all, parents can have more than one child without loving any of them less, so why can't a person romantically love more than one person?
Will this "sometime thing" truly make him or her happy?
This is reminiscient of what I wrote in my post regarding inadequacy: it's one thing to respect the other person's choice, but it's another to hang too much weight on that when you feel it's not the best choice for him or her.
I know he interests me too but he never chat me ㅠㅠ . But then I went to his bball game and then he talks to this other girl, and I know he likes him, but idk if he likes her now.......
In comments on my various posts on adultery, as well as in recent conversations with friends both new and old, the topic of loving two people (romantically) came up rather often.
But if you're in a relationship with someone that does expect monogamy and exclusivity (as many of us are), then loving somebody else at the same time does represent a problem.