One of the reasons it is so difficult to change our patterns is that nerve cells that fire together wire together. The more repetition, the stronger the connection.
To change negative thought patterns, we have to make new connections, and reinforce them until they are stronger than the old ones. Eventually, the old ones will actually disconnect. That is one of the reasons some people seem generally positive and some do not.
If you frequently have thoughts that "life sucks", those neural pathways have been reinforced and in order to form new ones, it will take consistent effort to notice yourself doing this and consciously change this viewpoint. This video has some really cool footage of this actually happening:
I like the comparison of mindfulness meditation and exercise. If you think about how ridiculous it would sound to explain weightlifting to someone who has no understanding of fitness - the wisdom of repeatedly picking up heavy objects and putting them back down. Until you have actually succeeded at doing this - building muscle - you can see why someone naive' to this experience would think it was ludicrous. Of course, most people do understand that lifting weights is one of the best things they can do to retain muscle mass and protect their bones and joints. It is also extraordinarily satisfying.
Mindfulness presents a similar misunderstanding: many people wonder "what is so useful about paying attention to my breath"? I think the comparison to exercise is useful, but it still doesn't get at what is so revolutionary about mindfulness practice.
The point of it isn't because it's good for you and will make you feel better, although this is true. It puts into question everything you do in the search for happiness. A great book describing this is Dan Harris' 10% Happier.
I write about human behavior, meditation, body awareness, and a variety of other things that pique my interest.