Feeling rejected is a very painful feeling and can take away your confidence. This can make it hard to continue with your life successfully.
It comes about because you have given too much power to your partner and become dependent on them for your happiness. If they leave you, you might decide that they did this because there is something wrong with you. This is always a mistake because it ignores the fact that your partner has their own insecurities and emotional issues.
If you have a pattern of feeling rejected you need to look to see if you are subconsciously rejecting yourself (feeling unworthy of love).