Careless

UPBRINGING.Period.

Some people justify their lack of carefulness with public property because people expect public property to get used more and wear out faster.

If I remember correctly the Rambam, in the fourth perek of Hilchot Teshuvah, says that stealing from the public is a sin that is very difficult to rectify.

this is partially because when you use a privatey owned object, you are usually known to its owner and therefore must answer to him and he will hold you accountable for damage etc; by a publicly owned object, however, there is very often no one to whom a borrower of said object is accountable to, which ehgenders a rather cavalier disposition towards safeguarding the object from potential damage or simply over aggressive / careless use of it.