Most of the relationships (marriage/ love/ friendship/ etc.) can be/ are broken when the third person applies the following formula consciously or unconsciously.
When two persons are mutually attracted towards each other, either the negative points of one partner gets invisible to the other or it is deliberately ignored and sometimes the other partner tries to change the habit of other to make it positive, for example: Cigarette smoking.
Thus, the point is they both become beautiful to each others, in few cases it might be acted by one or both the partners. Now, when the third person who is having similar status, qualities, appearance, etc. cannot easily beak this type of relationship because if the current partner is GOOD and the new person appears to be only slightly BETTER, people usually avoid experimenting. So, this good relationship can be broken by the BEST that too is not guaranteed.
BUT if in this GOOD kind of relationship some BAD/ UGLY/ POOR etc. will come then one or the other partner shall get distracted and even though it gets 2-3% space of the mind of any partner it would be able to overcome the 50% share of the mind and now at this stage of 50%GOOD and 50% BAD/ UGLY if GOOD leaves the partner and goes for a tour, on holiday, or fights with the partner and does not returns early or apologizes then BAD gets it 100% chance.
Here, the interesting thing is if the BAD goes away anytime he shall still have the doors opened for him and meanwhile his %share in mind too might have get increased.
EXAMPLE: You find a single copy of your interest in a bookstore and you purchase it immediately and you will move towards your destination visualizing how useful it shall prove to you, etc. GOOD partners' stage. Now, when at you home you start reading book after 50 or hundred pages (here you would not love to read similar interesting book) you notice that 2-3 pages have been printed upside down or they are very dark. I leave this example open ended, just imagine the psychological space these pages shall acquire in your own mind?