My 6 month old cries for hours on end most of the time picking him up stops him, but what could cause him to cry so often if he doesn't need anything?

His pediatrician can. Assess for medical ; neurodevelopmental disorders, with a history of when crying episodes occur, how long they last ; other specific info, physical ; neurological exams ; developmental screens. Learned behavior starts at 6 months in typically-developing infants. Get down on the floor to play with him, maintain a good schedule, ; re-direct him with short, direct statements ; questions.
Crying. At 6 months of age his crying may be due to teething.Try giving him cold things to sooth his pain. A frozen bagel half works well.
Learned behaviors... Children are smarter than we think. Your baby loves you and wants your undivided attention. He needs something--you! try playing or talking for set amounts of time and slowly desensitizing him to your brief absence, gradually increase the time until he's used to it; some babies just need more 1 on 1 than others; try to embrace this brief period of attachment before it's gone.