Why is it hard for women to put on mascara with their mouth closed?

Women were born with their mouths open and will die with them open.

Originally Answered: Why can't women put on mascara with their mouth closed? It's a nerve signal from our brains that confuses opening our mouths wider instead of the eyes, making us think that we'll do a better job at applying the mascara.

hahaha.... 

may be keeping their mouse open help them to keep balance 

Because when you stretch your face out like that it feels like you’re sufficiently pulling your face down to avoid smudges 

Try some mascara on.  Go on, do it!  You can take a shower straight after.  Once you apply it you'll understand.  When you open you mouth like that it pulls the cheek bones back a little and makes more space to get up under the lashes. so the mascara only goes on the lashes and not all over the skin.  It's only a few millimetres difference but it makes all the difference.

In evolution there was a good reason to open the mouth (in breath) when you open the eyes like that probably getting ready for flight, and putting on make up prompts/hijacks part of that evolved  reaction ("close nerves in the face trigger together" it doesn't prompt the in breath though you need the real thing (fright) to do that)