Diet obsession.. Why we see this in women so much?

I'm 5.2 and I weight 105 Lbs my BMI is about 18.68 some... Any way I been trying really hard to gain weight, because I always believe that men like curves and I dont have them.. I'm natural tinny and I hate it.. My question is that if in the real life Not hollywood life.. In the real life men like women with curves.. So why women try so hard to loose weight? Men love to see big and and legs.. Why women want to loose weight...? I want to gain weight.. I have a 4 months old baby and I was happy when I got pregnant and thought that would give me some weight and didn't.. I really dont like myself and I dont understand why women are so obsess about loosing weight.