If I lived alone, planning what to have for dinner would be much easier as I could buy just the ingredients I wanted. I live with my family, however, and my mum often ends up cooking lots of different meals in bulk. Some go in the fridge, some in the freezer. This often scuppers my plans on what I intended to cook.
When I do cook dinner, it usually depends on my mood. If I cannot be bothered, then I'll just stick some veg in the microwave and have it with tomato sauce. If I am feeling more creative, I'll see what is lying around the kitchen and then look at my list of recipes and see if I have anything that matches up.