You ought to be careful with the people you surround yourself with. I only recently discovered that certain mentalities are contagious. If you surround yourself with people that are constantly doubting themselves, speaking low about themselves, it's only a matter of time till you find yourself going through or experiencing such thoughts.
This is how some insecurities are formed. Some people project and reflect their own doubts and fears onto you. Therefore, you have the duty to protect your space, know what and who you allow in your space. Like the Bible says, two cannot walk together unless they agree. In the same way, let the people you associate yourself with speak and walk in a way that builds you so that when they fall short, you can build them too.
Allow positivity, happiness, encouragement, hardwork and love to flow through your circle. That way you will grow stronger not only physically but mentally and emotionally with the people around you.