They are attention seekers
While some people change their profile pictures, other people pay attention to the changed profile picture. To grab the attention of others, some people change their profile pictures frequently.
They get bored of their existing profile picture
Some people change the profile picture as they have got bored of the routine, due to which they cannot keep the same profile picture for a very long time.
They might be narcissists
It is not like all narcissists change their profile pictures often, but some narcissists who are much concerned about their looks and want to get the approval of others to feed their egos may go for changing their profile picture routinely.
They have a histrionic personality disorder
A person who has a historic personality disorder gets bored with the pattern and the routine and always looks forward to doing new things in life. These types of people may also go for changing their profile picture repeatedly.
They are confused
They may be inefficient in making a decision and always getting unsatisfied with their choices. This is why they may keep on changing their profile pictures now and then. (See Why some people have multiple Facebook accounts?)
They are insecure about their looks
Some people are not sure how good they look. They think that they are not appreciated for how much they deserve. Basically, they are insecure and want to get the approval of their looks now and then, making them change their profile picture periodically. (See Why do people care about looks?)
They are self-obsessed
Some people are obsessed with their looks, due to which they keep on changing their profile pictures frequently.
Why do girls make their Instagram private? Why do some people switch their Instagram account from public to private?