It is always a choice for people if they would like to celebrate Halloween or not. Some people do not celebrate it because they do not see the need to do so. For example, they may not have any children that can go trick-or-treating. Some people see this holiday as a complete waste of time.
The main reason why Halloween is celebrated is that of the belief that ghosts and other unworldly creatures do not realize that they are not among the living so they may walk free. By dressing up and following other Halloween traditions, they can be stopped. When people do not have this belief, they will not celebrate Halloween even if other people are doing it.