Relationships are interesting things... The reason the two of you started in the first place was because you were attracted to each other in some form or another. Maybe you liked his shoulders, maybe she made you laugh. You liked each other enough to form something solid and all of a sudden everything they did was perfect.
There's a point though when you start to see the cracks. You notice them doing something that had escaped your awareness before now. Love is funny like that, it can create blinkers so that you only have tunnel vision, their faults don't exist, until you let them.
When you love someone, you can forgive them for many things, but here's a few on which you should never compromise.
Maybe they told a racist joke in the pub when you first got to know them, at the time you thought they were just being ironic and laughed it off. After a while though, the racism jokes didn't stop. Muttering under their breath in a taxi, something awful about the driver. Making fun of the Korean guy in work, impersonating his accent. Racism is something that unfortunately, is ingrained into peoples attitude. They more than likely learnt it from their parents, grandparents, or other family member. It's hard to swallow but it's a very difficult thing to change in someone, so if you see it in your significant other and you think you can educate them yourself then good luck, it's gona be a long road ahead.
2. Lack Self Respect
This doesn't mean loving yourself, vanity, conceitedness. This means actual self respect for your own body and mind. Someone who eats crap all the time, who over sleeps, who fails to shower enough, who watches reality tv back to back is not investing in either their mind or their body. They don't respect themselves enough to utilise the complex gifts they've been given, like a working body and mind, instead they abuse them and are happy to let both turn to mush. Beware of these people because old habits actually do die hard.
3. Extreme Messiness
So I'm messy. There, I said it. Sometimes my laundry basket is so over flowing it resembles a game of jenga. I'm not talking about normal person messy, I'm talking about a cup that's been left out for 6 weeks, dirty underwear stuffed into the bottom of the wardrobe for months, plates stacked so high in the sink it's actually a danger. If you want a clean and happy life with this person, it's possible you're going to spend most of it cleaning up after them.
This might sound obvious, but cheating represents far more then just being drunk and thinking with your genitalia. Someone who can do something they are fully aware would crush you if you found out is someone with sociopathic tendencies. Because cheating is not just kissing or sleeping with someone else, a 'mistake' that never should have happened. With cheating comes lies, betrayal, deviousness along with a total violation of trust. People make mistakes, yes, but as Rachel from Friends' Mum always said 'Once a cheater, always a cheater' (Although that instance was different because they were actually on a break)
Possibly the worst rep one can get. Being known for your stinginess is like herpes, it's easy for people to believe and it's very hard to shake. They skip out on a few rounds, they ask to borrow money and don't pay it back, they eat others peoples food... If you're in a relationship with someone like this, you will find yourself making excuses for them. It might even get to the point where you pretend they bought something, just to salvage some respect for them.
Again, it might go without saying for some people but addiction isn't always that obvious. Someone with substance abuse can be extremely sneaky and all sorts of lies can come with it. If they're not aware of it, it can be worse. Denial is a scary thing when you're faced with it first hand and unfortunately all the love you give them won't sober them up.
If they hit once, they will hit again. They don't have to directly come into contact with you to be violent either. Smashing a few plates or punching a wall is just as dangerous. Issues with violence need to be acknowledged and dealt with professionally or else you need to get the fuck out.