Kayzee

Interestingly enough, Samhain is supposed to be a somber holiday of reflection, not a cheerful celebration with candy.  The barrier between life and death were considered thin because this was the time when people decided which of their flock would be slaughtered for food over the winter and which would be kept for breeding in the spring.  A wrong choice could mean ruin for the family, which only made the decision that much more serious!  With all that death (from the slaughtering), the barrier was considered super thin, so people were warned that various types of spirits might visit, so be polite!

(Isn't history fun!?)

It's not only just because of death though! It's also often explained as the start of the dark half of the year where days are shorter and colder. There is another day where the barrier between worlds is thin too, and that's Beltane or May Day which is the opposite day to Samhain!