The phrase "go to your head" is generally used to refer to a person who is acting in a way that, for him, isn't normal. Most often, this means adopting an attitude of pride or arrogance. Things that are often said to "go to your head" include fame, money, praise and power. The phrase can also apply to an advancement in social or professional position. While the origin of the phrase is unknown, it is a commonly used English idiom.
The idea behind the phrase is that a person allows a change in circumstances, often sudden and almost inevitably positive, to change his behavior, beliefs and actions. For example, someone who is normally humble and generous might become arrogant and self-centered after winning a large sum of money or suddenly becoming famous. Behaviors characteristic of letting something go to your head might include demanding to be treated better or differently than others; expecting recognition; thinking that rules need not be followed; and ignoring the rights, needs or feelings of others. A person who has let something go to his head might also come to expect a different standard of living and might consider that old friends or family are no longer appropriate to his new lifestyle.
Often, this phrase is expressed as a directive — specifically, "don't let it go to your head." This means that the speaker is telling the person that he shouldn't become overly confident or cocky because of whatever he is being given or granted. Used thusly, it is a warning against taking on attitudes that are unbecoming.
In literature, in movies and on television, letting something go to your head is often part of a morality tale. In such plots, the person who lets something go to his head almost inevitably loses friends who will, eventually, prove to be more loyal than the new friends he has made. Usually, the character who has abandoned his friends will be made to realize his mistake. Normally, he will then repent, make amends and reunite with the original friends.
Alcohol is commonly said to go to someone's head as well. In this case, the phrase still denotes a change in behavior. Rather than meaning that the person behaved badly, however, it really just means that he got drunk very quickly.
Other related phrases include "getting a swelled head" and "getting a big head." Something that "gets in" or "gets inside" someone's head is quite different. These phrases are usually used to denote something about which the person cannot quit thinking and which causes him to fail or become overly self-conscious.