Reasonable & Unreasonable ParentsThere are the parents who are too strict with their rules. In order not to pamper their kids, they establish something short of a dictatorial reign at home. They become too controlling, too interfering and also unreasonable in their punishments and rebukes. But this is not really the case with most parents.
Most of them simply treat the teenagers as they used to treat them as children, which most teens seldom appreciate. Where such types of ‘normal' parents go wrong is when they imagine their adolescent boy or girl as an old child, not as a young adult. They think it's just right for them to expect him/her to comply with what they say and to look up to the old values, conventions, rules, etc. They find it strange and get upset when such things don't happen with their boy/girl. Parents take a while to change their approach to the kids, as they initially expect the kids to change themselves, which is not exactly likely... And any type of incorrect approach to the kid is enough to spark off a rebellion.
Some other parents keep pressurizing teens in a particular field [mostly academics], without considering the dreams or ambitions of the teens first. Such parents consider their decisions to be the more ‘practical' one, and this creates a rebellion, as teenagers hardly consider their dreams as ‘impractical'.
Values and Rules
But most of them simply start questioning them once they're out of their childhood delusions, because children [< 11 years] don't really know what is right and what is wrong. They know what their parents will like and what they won't. Children are taught to be honest, kind, polite, clean, etc. but often they aren't taught why. No matter how many times their parents might have talked about it, they might still be ignorant about why they should follow rules and give importance to values. They simply obey in order to avoid rebukes or punishments. So in adolescence, they try to understand the values for the first time... they begin to question, experiment and discover for themselves, because that's most probably the only way in which they'll truly learn the importance of rules, conventions and values.
The problem is that during that process, parents try to enforce their rules even more strictly [I'm not saying it's wrong, but it does result in some teens defying them even more] and some parents think it's just arrogance or disobedience [That's definitely wrong though] when a teen asks ‘Why?', which generally disturbs the relationship and hence, rebellion.
IndividualityTeenagers start searching for an identity for themselves and struggle to maintain their individuality. At this stage, they start asking questions like "Who am I" and more than often, they'll receive unsatisfactory answers from their parents. This is the time when they become aware of their shortcomings and also those of their parents. Sometimes, they may become rebellious in order to protect themselves from acquiring the weaknesses of their parents. This usually occurs if the kids are embarrassed by them at a couple of instances. Teens also adopt various personalities, trying to find one which will give them a hint as to what sort of person they are and they also look for role models, though most of them don't consider their parents in that aspect.
Some teens, desperate for an identity, try to be different from the rest and end up getting in trouble with their parents for their weird ways. Every so often in this process of identity and individuality search, it hurts a teen when his/her parents make fun of a personality he/she tries on. This is when kids learn to be shy and begin to hide their preferences from their parents and try to even keep away from them. Peers of course, are more accepting at this stage and a teenager usually doesn't mind being teased by a few of his close friends. This is probably because the criticisms are mutually shared between them.
I've noticed so many drastic changes in the personalities of my childhood friends and myself, so I know for a fact that much of the person's character is decided by how much he/she experiments and changes his/her traits during adolescence.
Teens also wish for privacy, as it seems to give them a sense of freedom and also helps them to test things out for themselves without parent input. Teens long for the liberty that adults possess, but are also dependent on their parents, emotionally for love and care and in many other aspects as well. It creates some frustration in them, as they are midway in life and can be neither children nor adults.
PeersThe negative influence of peers is always there and is a great matter of concern. But personally I don't think most cases of normal rebellious teens are due to peer pressure. Sometimes, when what peers value is at a contrast to what parents value, the teenager may get pretty rebellious, as he/she will most certainly support the views and opinions of peers.