Why people sabotage relationships

The classic sentence that an avoidantly-attached person will say to their partner after a few beautiful weeks, a few mini breaks, a few lovely times, they'll go, " this is feeling a bit too intense," and then they will withdraw. They'll pull away from a relationship because it's getting to intense. And really, what they mean is I'm so affraid of love.
And the way thay I deal with my fear of love is to pull away. It's not that the avoidantly-attached person doesn't want love. They want love. They're just terrified of it. Why are they terrified of it?
Because it didn't go well for them in childhood. Because love is a threat. If you've grown up in an environment where love was not possible, where relationships between caregivers and childeren were not safe, you will have defended yourself against the risks of disappointment.
You will have insulated yourself. And then when love comes your way in adulthood, what you will do against your knowledge, against your conscious knowledge is make sure that love does not succeed. You will sabotage your own chances of a succesful relationship.
When it really becomes clear that love may work. That's when the traumatized attachment lover get's their sticks of dynamite out and starts to blow the foundations of the house up, so that they can return to the suffering that feels more familiar.
What you need to understand is that in love, we don't look for what will make us happy. Absolutely not. We look for what feels familiar. For many of us, what is most familiar is a sense of not being loved properly, a sense of not being sure where we stand. A sense that someone may threaten us. A sense that someone may abandon us. That is our true home.

Alain de Botton