Firstly, don’t cycle in jeans. They restrict the movement of your legs when mounting & dismounting and if you cycle for even an hour in them you’ll probably get a chaffing injury and be on the bench for 3-4 weeks waiting for it to heal. Wearing a regular pair of shorts can also cause a chaffing injury. Same goes for cotton shorts: they’ll get sweaty remain wet and you risk a chaffing injury.
So you definitely need a decent pair of cycling shorts. But you won’t get them on Amazon; I tried. I’ve had good results with Fox’s Ranger Lite Shorts which are sized in 2″ waist increments. These are mountain bike shorts with padded cycling shorts tacked to the insides. Just snip-out the inner cycling shorts and use them WITHOUT the outer shorts (shown in red in the picture to the side) which are hot & sweaty on long cycles in the summer time. The outer shorts are still quite smart looking though and can be used as a pair of casual shorts! They’re not cheap, but I got mine on sale because the colour red apparently wasn’t selling. But I only use them without the outer shorts anyway, so colour was a total non-issue. And buy (6) pairs if you cycle 6 days a week. Do not re-use unwashed cycle shorts or you’re back to getting potential injuries. I bought (6) pairs because I didn’t want to have to wash them more than once weekly. I highly rate Fox’s Ranger Lite cycle shorts.
You can buy them from Fox Racing HERE