they are competitors - the same job, the same championship, etc.
they are straight-up enemies - one them wants to buy the land to build a resort, and the other wants it to get turned into a nature sanctuary. one is a defense attorney, and the other is a prosecutor. they’re soldiers on opposing sides of the war, etc.
they already won - they inherited an estate that should have been the other’s, or one got the job that the other was hoping to be promoted into, and they are seething.
they broke their heart in the past - maybe there was a good reason behind it, or maybe they were an idiot then and realize it now.
they did the other wrong in the past - maybe they wronged the other’s family member or friend.
they have has trust issues - their last relationship ended in a terrible betrayal.
they have intimacy issues - maybe because of a traumatic past, they feel too vulnerable if anyone knows about their real emotions or weaknesses. maybe they’re a psychic, a behavioral expert, or just the one person who can see right through the other and it makes them uncomfortable as hell.
they don’t trust their dating choices - they’ve gotten involved with several losers in a row, and they’re taking a break, maybe a permanent one.
they believe no one can truly love them - they made a terrible mistake in the past, or they’re not attractive in the conventional manner.
they are good friends - they don’t want to risk ruining their friendship by taking it to the next level.
they are co-workers - they don’t want to make things weird at the office, restaurant, or wherever they both work.
they are freaked out by who they really are - a werewolf, vampire, ex-con, spy, etc.
it was supposed to be sex only - they were going to be friends with benefits, or maybe one of them is an escort. falling in love goes against the original arrangement.
their love was supposed to be fake - they were pretending to be in love or married, or they entered a marriage of convenience.
they were faking it at first - they made a bet that they could bed the other, or pretended to like the other so they could learn their darkest secrets.
they have moral qualms - one of them is the other’s boss, their teacher, their captor, ex-girlfriend or their best friend, and that gives them pause.
they are struggling with personal issues - one of them is grieving the death of their significant other, best friend, their child, or they have PTSD from battle or some other terrible experience.
their love is forbidden by others - it’s against the rules, spoken or unspoken, of their family, organization, community, or religion.
they are into something that the other doesn’t like - it could be an addiction, like alcohol, drugs, gambling, or a profession, like boxing or defending corrupt clients in court. whatever it is, the other wants no part of it.
their cultures clash - one of them is a rock-n-roller and the other is a business executive, or they love the city life and the other is all about small-town living.
one of them is promised to someone else - maybe the wedding’s already planned and everything. they might have reasons for going through with a loveless marriage.
one of them is married to someone else - it is some weird legal, not-consummated arrangement, or the other ran away long ago because their spouse was an abusive asshole.
they are infatuated with someone else instead of them - they don’t realize yet that this other person is awful.
they are interested in someone else in addition to them - they’re having trouble making a decision on who to be with.
their time is together is limited - one of them is going abroad to study in the fall, their work visa is almost up, or one of them is about to go on a space mission to another planet, never to return.
being together would require a big sacrifice - one of them would have to say no to their dream job or they would have to move away from their family.
they are a danger to the other’s safety - they’re a spy, vampire, drug lord, gang leader, etc.
they think they’ll only make the other unhappy - they’re dying of a disease, suffering under a curse, can’t have children, etc.
they are a suspect - in a murder or robbery case.
they have a reputation - a materialistic douche, a heartless womanizer, a stone cold killer, etc.
they have terrible first impressions of one another - they might have gotten off to an awful start by arguing over something.
they love their freedom - they always saw themselves as the carefree type who would never settle down.
they don’t want distractions - one of them has a very important job to do, and they can’t afford to get sidetracked.
