I have been married 14 years to this man and we have been through some very rough times together. But I am very happy that despite everything that has happened, we are still making it work. Despite all these years, I cant still say what can destroy a relationship. Yes, there's infidelity, loss of trust and lack of passion and a lot more but if one is willing to forgive, the relationship can still be mended. But, only if both parties are willing to work it out. It's easy to say I forgive you, but if you bring it up again every time you fight, then where is the willingness to forgive and forget? Or if the party who made the mistake has to take all crap from the other party just to earn the forgiveness, sooner or later, he will tire of it. So, both must be willing to work it out and both give an effort to make it work.
And why o why that picture? Well we are surrounded by sharks who are might be willing to attack us, given the chance! Thanks God they are in an aquarium and we are protected by that glass wall. So okay, its a stretch but relationships are like that. You must protect it, and surround it with love. But if one strays, decide if you want to make it work and do it. If not, just let go.