[rhodri]

My opinion is that most of the bad habits of inexperienced programmers are just manifestations of more general youthful hubris and inexperience in the nuances and complexities of life.

I'll add to the list the most extreme form of 'not written here syndrome': 'I didn't write it syndrome'. The programmer sees themselves on missing out on the fun of solving a task by using a library (or reading the relevant framework docs). When experience teaches you that the real misery comes down the line when you need to support your hand-rolled physics engine lacking in tests...