I am not one of those people who claims that there is no difference between the two parties. Major differences exist. I do not wish to see Romney in the White House. He and Ryan are at best habitual liars.

Obama 'ended' the war in Iraq like promised. He also escalated the war in Afghanistan like he promised. If I murdered 10 people instead of 20 I would still be a murderer. As long as American troops under his command are killing civilians* I can not vote for Obama as my representative. Romney would probably get us engaged elsewhere with an even higher death count. That does not sway me towards accepting a man who supports a war that I do not.
There are other reasons, but that one alone is enough for me. I will not volunteer to be represented by such a man.
Will you support someone who doesn't represent you?
* Which they are. Some might be 'collateral damage' but others are documented as unwarranted massacres.