I love a cruise! You get to go to lots of different places without having to travel there. You unpack once in your room and each day wake up in a new location.
Sea days are very relaxing. There is often entertainment put on, which you are free to participate in or not as you choose, from pub quizzes to exercise classes to lectures to wine tasting. I love just grabbing a cup of coffee and finding a nice quiet spot by a window to read. I'm not sure what you mean by space from fellow passengers. Even when a ship is full there are always quiet spots to sit, and plenty of seating in the different venues.
When you're in a port you choose what you want to do. You can take part in an organized excursion, which is often a great way to see the place or have a new experience, or you can just get off the ship at your leisure and wander around. Or you can stay on the ship if you choose.
I've had some amazing food on cruise ships and some more mediocre, but that's no different to being on holiday in one fixed location. I like how you can choose what you're in the mood for - lots of different choices in the buffet area or a sit down restaurant meal in the dining room.
One of the things I like the most is the evenings. There are usually several bars with different types of entertainment. I can't think of anything much better than hanging out with my husband in a bar having a few glasses of wine and watching a band, and then only having to go down an elevator to get back to our room! Some people like the theatre shows in the evening, but that's not our cup of tea.
Occasionally a ship will be pretty rocky in bad weather, and people do sometimes get seasick. Personally I've never been sick but there have been times when I've felt a little nauseous. But apart from that, I can't think of any reasons not to enjoy a cruise.