There are a number of factors that can impact the degree to which any individual will be conformist in any given situation. However, people generally conform for two main reasons. One is personal and the other is more generic.
On the personal level, people have different personalities. These appear to be inborn traits. Some people are simply more likely to conform than others. Some people naturally want to ingratiate themselves more with other people while some are more inclined to be individualistic.
That said, all people feel some degree of compulsion to conform. We tend to want other people to think well of us. We do not typically want to be the one person who is different and is therefore left out of the group. To be sure, this depends to some degree on our socialization. Some people may have been socialized by family and others to rely on themselves and not to worry about social pressures. But most people do feel social pressure to some degree. It is simply more comfortable for most people to go along with the crowd than to go against it, even if they disagree with what the crowd is doing.
Please follow the link below for a more detailed discussion and some more factors that affect when people conform and when they do not.