'Why date him when he disgusts you so much?'

What's the best way to tell someone that their behaviour is obnoxious? I hate the way my boyfriend talks, dresses and acts around me, as well as other people. How can I tell him this? Should I?
Why are you with this person if you find everything about him so disgusting? Also, if you can't open your mouth and speak about it, you are doing him as well as yourself a disservice.

I dated a girl for a month, after which we decided to break up because it didn't seem like there was anything possible between us. There was no spark, we didn't enjoy going out much, and didn't even chat much when we were together. Even our friends thought it seemed a little forced, so we ended it and things were fine.
A month or so after we split up though, we began chatting casually on WhatsApp. This continued for a while until we began chatting with each other a lot, and actually had a lot of fun. Now, we chat all day, and tell each other a lot about ourselves, which we didn't do when we were in a relationship. She says we should get back together and give it a chance. I don't know if this is a good idea because if we don't connect again, it will simply mean sadness for us both. Is it worth it?
Everything is worth it if it makes you happy. To give up on the possibility of a great relationship simply because it didn't work out once is not reason enough. It takes time for people to get comfortable with each other, which probably explains why the two of you didn't get on like a house on fire immediately. Maybe you both had different expectations and starting out with none whatsoever will set you both free. Stop over thinking this. If you like this girl and enjoy her presence in your life, why wouldn't you want to be with her?