How Do You Know?..

How do you know what the truth is?,...after so many times that you've been lied to? There are alot of people that I know that are in relationships and don't have trust for that person,..well if you don't have trust in that person then why are you still with them?
You know after a couple of times your spouse lied to you,you begin to loose their trust,but when your friend lie to you,you soon forgive them,..why is that so? I feel that if you can accept your friend for lying to you then why can't you do the same for your spouse,...or the person that you have relations with. Not everybody is perfect you know,everybody has their slip ups and their down falls,..but thats life you know,..but thats also how you find out your right from wrongs. So how do you know what is the truth and what is not?,..determine the truth by paying attention to your surroundings and talkit out in your relationship as you do in friendship.