1) "You have so much history together, you're for sure going to get married" How about...no. A lot of history does not equal getting married, especially if that history is bad. Sometimes two people just don't fit, and it doesn't matter how long you've been together.2) "Eventually they will change/grow up" I'd say that most people don't change. As for growing up, it may take quite longer than you think/never happen.
3) "Just don't say anything. Wait until they text/call first" If you have something to say, say it. Also keep in mind sometimes your s/o may not know WHAT to say. Silent treatments are a waste of time, just express your feelings.
4) "They're such an a-hole. You deserve so much better" Let's face it, no one else but you truly knows what went down and how you feel in your relationship. Also, sometimes friends don't realize how much you really care about your s/o. Everyone makes mistakes, and shit is always gonna happen anyways, with anyone. At the end of the day, only YOU know if your relationship is right for you or not.
5) "Be careful, things are moving so fast between you two" Please, tell me, when did your friend become an expert on how long to wait before taking "serious steps"? The way I see it, if you're meeting the parents, or moving in with your s/o...etc, it means you're A)falling in love and B)wanting to see if your relationship REALLY works. Only YOU can know when you're ready, and no one can or should judge the speed your relationship is moving at.
6) "There's plenty of other fish in the sea" True. But finding someone compatible to your personality, who makes you laugh, and with whom you have great sexual chemistry among other things, can be really hard to find. Sometimes working through disagreements, communicating honestly, and compromising, can definitely be worth it. Also, no one likes break ups.