Why do the neuron pathways decussate? I learn about the brain right now, and there are crossing of neuron pathways everywhere; in the thalamus, the medulla oblongata, the pyramidal tract... And I don't understand why? What is the reason to connect - for example - the right foot to the left part of your brain?

Crossing or decussation is a lot more robust against wiring errors than their seemingly simpler same-sided wiring counterparts. See here for the research carried out to establish this and for more detail.