Why is breastfeeding hard?

I'm pregnant with my first child and I've always heard breastfeeding is hard in the beginning. I guess my assumption is that it's painful at first but your body soon adjusts and it no longer hurts the same way. But again, this is just my assumption since I've never done it before.

Are there other reasons that it is very difficult at first? What was your experience like? And can you describe the pain associated with breastfeeding in the beginning? I just want to understand what the challenges will be before I attempt this on my own in a couple months.