There are many reasons may be considering downsizing...
MORE TIME. Fewer rooms or smaller spaces will cut down on the time you'll spend on cleaning and maintenance.
LOWER UTILITY COSTS. It costs less to heat/cool a smaller home. Less square footage decreases the amount of energy expended.
LESS STUFF. If there is no place to put it and store it, you'll be less likely to buy it and accumulate it!
MINIMIZE STRESS. Less responsibility, smaller workload, smaller investment - added together, they all reduce stress.
Remember, moving isn't easy, it presents a significant change that while exciting and positive can be both physically exhausting and emotionally draining. If you're having a difficult time making decisions about what to keep - what to part with; take a break, talk to someone, ask for help. Remind yourself throughout the process that your goal is not to get rid of everything you hold dear, but to simplify your life.