There are lots of reasons why people will flat share or house share; we can group them into 6 main reasons.

     Living in a flat share will reduce your living costs (and create income if your a landlord).
     Some professionals work a long distance from home, so a flat share near work is an ideal solution.
     People sometimes house share just to live with other people and be sociable.
     Live somewhere nicer, if you share a house then you can normally live in a nicer house than if you lived alone.
     Getting on the property ladder, so people take on a lodger to help with the mortgage.
     Your a student and going to university and don't fancy living in halls of residence or a bed-sit.