In a positive relationship, you are inspired to internalize your partner’s positive qualities that you are lacking. This is how synergy (1+1=3) happens between two people. If you spend long enough time together in this manner, you would eventually internalize most of the positive qualities that your partner has. Even if that person goes away or even dies, it’s as if you already have him/her within you.
In a negative relationship, you feel relieved not to have to deal with your own weaknesses because your partner can take care of them for you. So, those weaknesses become even weaker. 1+1 becomes 1. The longer you spend time together, the weaker you both get. You become fearful of losing your partner. This fear would eventually lead to feeling trapped, which is often expressed as anger toward your partner.