I cruise solo and take typically 5 or 6 cruises a year, most often on Royal Caribbean. I choose my cruises based on destination and to some extent cost.
My recent cruise on Mariner was chosen because I like Rhodes as a port and had not been to Istanbul before, combined with a single supplement of only 50%.
Great cruise, as usual; missed lobster, but I understand the cruise lines are all cutting corners on cost. Weather in the eastern Med was perfect as usual, and company at my dining table was pleasant, being from a number of interesting and different backgrounds. When travelling solo it is good to choose a large table.