why do some people think they're ugly when theyre not?

[All My Intensity]

[marianne]

low self-esteem

[Inmyhead*]

They are just seeking attention?

[DramaPajamaLlama]

Generally it's because they compare themselves to others much more than they should. Also the people they compare themselves to are people like beautiful celebrities, who really have an entire crew to fix them up every time they leave their houses. Also, they look at each of the parts of them in so much detail, they see every single fault of them and add it up, thinking they're ugly.

[wolverine]

I fall inlove with personalities not with looks at all

[Fake Genius]

some (only some) need an excuse for plasticising themselves.
http://www.shockmansion.com/2014/03/27/v...-sex-doll/

[Boss]

low self-esteem

[Robman #YOLO]

If there self conscous

[largehobbies839]

Because it's easy to focus on your flaws

[DellaRobbia]

All answers above are correct: mostly it's that people have low self esteem.
Some do just seek attention (i.e. skinny girls complaining on social media about how fat they are to get people to tell them they're beautiful and good looking people complaining about how "ugly" they are to get people to "cheer them up" by telling them they're beautiful.)
And then others do it just to make a stronger case for them getting plastic surgery.
All in all, a lot of people spend a lot of time comparing themselves to others, and it gives them low self esteem.