Several factors come into play for me.

First, I'm on my feet and active all day, except for my two breaks and lunch. Sometimes by the end of the day I'm just plain tired--too tired to pedal effectively. There have been times I've had to granny gear home on an upgrade so miniscule it barely qualifies as even a false flat.

Generally, these things make it worse, roughly in order of most important to less important:

• Not enough riding--if all I do is commute, or my commute is my toughest ride, there's going to be trouble. I need two or three "training" rides a week that are longer and harder than my commute in order to stay in shape for my commute.
• Insufficient sleep through the week
• Not eating right in general, or not at all on my afternoon break
• Not watching my water intake. My system naturally runs on the dry side. I have to force fluids 365 days.
• Nervous tension and anxiety. Funny how the mind gets in the way of the legs.

There may be other stuff I haven't picked-up on yet. And of course, these are the things I've noticed that increase difficulty for me. Things may be completely different for you.

Oh, and there is the wind. It's generally a tailwind to work and a headwind home. It's brutal in February and March.