When a guy has been rejected by his partner…his insecurities come back to the surface
If They’ve Made the Wrong Choices
Whether or Not They’re Good Partners
Having (and Showing) Emotions
Is his dick big enough
Where They Are in Their Careers
Is he attractive enough
Not being successful enough. Attractive enough. Talented enough. Man enough. You name it. I think in general it just comes down to the way I unconsciously, and sometimes even consciously, compare myself to others and this overwhelming feeling of not being enough for anyone. But then I remind myself that the eight-year-old boy inside me who is still so hurt from being bullied, and teased, and who came home so broken, in reality has so much to offer this world, is loved deeply by many, and is more than enough. And when I can go there, I feel at peace. – Justin Baldoni
dave94015 
Insecurity born out of our our childhood vulnerability is habitual and hard to shake off. Then, rejection of any kind triggers our previous reliving of trauma. Me too.
Comparison is a joystealer
It’s not only guys that feel that way (Except about the penis size). Though with girls it is usually “am I too fat?”
women have many body issues which contribute to their insecurities