Plastic surgery is their personal choice. If someone wants to become more beautiful by plastic surgery then let them.
Anyway beauty resides in the eyes of watcher.
This depends on many things. Including, mental manipulation, cultural values, self-esteem, spiritual de-generation, psychological problems, emotional 'blocks' etc...
Each person is different and may have a different reason why they don't like their appearance. It may be related to, for example, trauma, low self-esteem, social norms, dysmorphic disorders, etc.