When you are in a relationship with someone, your lover will become you best friend. Most of the time that results because they are usually the ones you talk to the most so eventually the two become close and they find themselves expressing their feelings to each other without even realizing that they are becoming closer every time. But, if you aren’t married then you still probably go to your best friends and tell them everything going on between you and your lover.
They know your deepest darkest secrets, and you know theirs as well. Depending on how long you all have been friends, if it has been a long time then you also probably know everything about them, and the way they think, etc.
They stand by your side and take up for you no matter what. Its a great thing!