Practice isn't a way to change ourselves into who we want to be - it is about paying attention to who we actually are. This is not to say that change does not occur, but rather that we mustn't pick out some aspect of that change and try to hold onto it. After we sit for a while, perhaps we find ourselves to be calmer, or maybe even blissful, or seemingly imperturbable. But as soon as we seize on any one experience in the midst of practice and say to ourselves, That's IT, that is how I want to feel when I sit or how I should feel all of the time, then we make ourselves rigid and brittle, by trying to hold on. Instead of being open to the ever changing flux of our experience, we try to hold onto one particular feeling, and then subtly, we're on our guard, lest something perturb our inner peace. Then life, instead of being our teacher, becomes our enemy, always intruding, always threatening our peace of mind. All because we've finally "achieved" some change or insight in our practice.
True practice is always a willingness to face life as it is. Yesterday, a friend called saying that her 8 month old son had suddenly died from meningitis. If you think practice is about achieving an imperturbable calm or bliss, what will your practice be for you when you have to face something like that?