Why do we willfully and consciously engage in self-destructive habits while ignoring our better judgment?

we don't think about its major health injuries and at the same time we thought about the difficulties that comes when we quit bad habits and hence its become a serious trouble 

 because we don’t rationally weigh the risks vs. costs.

we as humans want immediate  pleasure  and everything that makes us feel good.  as for most common eg-

When we smoke, we think it’s too hard to quit, too painful over the few weeks it takes to quit (cost), but it’s not properly weighed against the risks of not quitting (major illnesses, suffering for years, early death, incredible expenses for cigarettes and hospitalisation, etc.).