Seeing how it really is and seeing it the way you want.

In this world people have opinions, opinions with mostly everything and basicly it has two ways which is seeing it the way it is and seeing it the way you want or in otherwords seeing the truth and seeing delusions. For example drinking alcohol, a teenager might say that it's totally ok and he likes to drink because it made him feel good. On the other hand a religious person would say drinking is bad and can lead to commit other sinful actions; totally different views about drinking alcohol. The thing is each person would think that what he thinks is the right thing and that's the HARD part. How could one knows that what he believes in his head is the truth? and in contradiction how could he knows that it's not a delusion? I would say each one would have to find an answer to himself.