Why do friendships get destroyed due to relationships?

I think the reason why friendships get destroyed due to relationships is mainly because less time is given to that friendship. Take for instance, you had a best friend before you got into a relationship with someone else, you spend more time with your new partner and less time with your friend. Over time, the friendship begins to suffer and if your new habit continues this way, the friendship does gradually.

Not many people are able to manage friendships when in a relationship with someone else. It takes a level of maturity for someone to understand that one doesn't have to suffer because of the other.

In a loving paired relationship, whether heterosexual or otherwise, there is often an intense desire for it to be exclusive and excluding. The partner cannot bear to think of his/her love bestowing time and attention on another. Worse, that s/he may be enjoying that time as much as with the lover. Even with caring and thoughtful individuals this intensity can seriously affect the feeling of well-being such that they cannot stop themselves making barbed remarks or, at worst, putting limitations on what their partner is 'allowed' to do or where they can go, how often and who with. This possessiveness is part of many paired relationships and in an extreme form, is unhealthy.

The forming of a loving relationship is heady and exciting: the thought that they are so loved by an individual. Even limitations placed upon them by the partner is seen as an expression of their deep love. It actually takes a very mature individual who can love with all their heart, remain loyal, yet enjoy their own friends and allow their partner to enjoy his/hers. This can, and often does, extend to family members. Does s/he love his/her parents too much? Oh, love is such a complex affair...but you have to get on with it and rise above pettiness.