There is a school of thought that says that we should say and do everything that we think and desire without guard because that is the healthiest thing for us to do as we are free to give expression to that which is within us. There is another school of thought (it is called the school of good sense through experience) that says that once we have said something, it cannot be unsaid, once we have done something it cannot be undone, once we have broken a trust, it cannot be unbroken; it can only be mended, but even in that, there will always be a scar or a mark that reminds us of the consequences of unguarded actions.
I believe that as we process any conversation, any setting, any action which we are a part of or which we are simply observing we have a multitude of thoughts and possible scenarios that run through our minds as we work out which one would be the best scenario for this given conversation, setting or action. Obviously since there are many possible scenarios that we process, we are unable to say or do all of them because they are often opposed and ultimately we would have to choose one scenario because we can’t simultaneously do or say all of them (once we have said or done something, the situation then changes and we need to process a different multitude of possible scenarios). The scenario that we select, the one that would often be the first one that comes to mind would be that which we spend most of our time considering, the one that we would consider most acceptable for us. We effectively train ourselves how to respond and react in any given situation.
We guard our thoughts (what we think about, what we listen to, what we watch) because our thoughts become our actions and our actions come to define us as they become the means by which we will be perceived by the people around us who will often bind us to that definition long enough for us to start believing that we are the person they think we are. It is often only when somebody comes along and offers us an alternative possibility as to who they can see us being that we begin to believe that about ourselves. We give others the power to influence our lives and our self-image, and often we give them too much power.
Paul calls the believers to allow God to influence their lives, he calls on them to allow God to be their first thought so that the first possible scenario that comes to mind in working out what to think or say or do would be one of building up instead of breaking down. With God as our first thought (and our last thought), we immerse ourselves in the love of God that is beyond understanding but is seen and testified to in the healing love of life that becomes a part of all we think and say and do. Think on the things of God and as you meditate on it day and night, allow it to become your first thought in everything.