Sometimes you just go through phases where you feel more or less motivated to get on that bike and ride to work. I think it's normal to have a love/hate relationship with your bike (I used to love riding to work, now I hate fighting the traffic). Don't beat yourself up over it. Maybe you're just a little burned out on riding in traffic. Is there any other way to get to your destination that involves less cars (or maybe you could go in a little earlier or later to avoid a chunk of traffic?).
Maybe this will work: think of how much good you're doing for your body and the environment when you pull on those bike shorts and shoes and head to work. Your body is healthier, your mind is healthier, and who knows; maybe you even inspire someone at your work that you don't even know about. Nothing is better for the body/mind/mood than some fresh air after a long day's work.