In normal life we are usually moving all the time, so we don't get to see the nature of the body, that there are pleasant and unpleasant feelings arising. So then we form the view that the body is always pleasant, because every time there are unpleasant sensations, we move. So, by actually having to sit still we get to see the nature of the body a little more clearly. And hopefully then we will develop more capacity to not react to such arising and passing of sensation.
Yes, the mind can think anything. That is Mara, ignorance. It wants to attack you, it doesn't want you to sit there and uncover your deeper potential. So it will send the most stupid thoughts you have ever thought of at you, anything to pull you off balance, anything. It becomes quite interesting sometimes what the mind will think, and if you have a bit of what I call cosmic humor, just smile, like the Buddha's smile. Mara is trying to pull you off balance. Mara doesn't want you to uncover your potential and lessen your ignorance. So, Mara will throw a lot of its powers at you to stop you doing what you are doing. One of the first things it will throw at you is doubt, "What am I doing here, I could be at the beach enjoying myself!" So, we just try to have a little bit more equanimity towards thoughts and understand that after a little while if we just bring up mindfulness and note it as it is and try not to think, "where did this come from," or "why am I thinking this?", we see that a thought is just a thought and we don't get so involved in the contents of the thought or take to be "me."