I do not beleive in zero tolerance when it comes to relations. Zero itself is a symbol of emptyness, presense of anything. When we have already established a fact that no one is perfect then zero tolerance would cross out everyone around us if we were to follow this criteria. At the same time, I beleive three strikes out is too flexible a choice to adopt in a relationship. So the middle choice is two. Give everyone a second chance but if you give them more than two chances then you will become the victim at that point. Because if you think you are giving a person three chances before you move on then think of it from the other person's perspective. They hit you not once, twice but also got the chance to strike thrice then you are nothing but merely stupid. So even if it works out later, it would be a relation between a stupid and a selfish. Who could hurt someone again and again and again, none but a selfish person indeed.