Humans are very disappointing creatures. Eventually, all of us succumb to selfish whims, bad moods, or exhaustion. When this happens, someone close to you is very likely to get hurt. As a matter of fact, the closer they are, the more likely that is to occur.
If your level of sensitivity is quite high, being friends with someone or being a member of a family requires that you get hurt frequently. On the other hand, having very low sensitivity means that you probably hurt others frequently (even unintentionally).
The bottom line is that, if you wait long enough, the people will hurt you. Mostly it will be emotional blows, but it can be physical too. If it's only emotional pain (and it usually is), you are supposed to accept it. And what else can you do? Short of becoming a recluse?
You have to interact with people so hurt is inevitable.
Yes, there will be good times. Laughter, love, friendship, secrets, inside jokes, relaxation...this can make a betrayal even more hurtful. The longer the space in between hurts, the more the next hurt hurts.
They won't mean it. They probably won't even notice it's happened.