The shifting direction for front and rear shifters on bicycles is different due to the design and mechanics of the derailleur systems used for each.
Rear Derailleur: The rear derailleur is responsible for shifting the chain across the rear cassette, which typically has multiple sprockets. The rear shifter controls the movement of the rear derailleur. When you shift to a higher gear (harder gear), the rear derailleur moves the chain onto a smaller sprocket, and when you shift to a lower gear (easier gear), the rear derailleur moves the chain onto a larger sprocket. The rear shifter has a lever or button that you push or pull to move the chain in the desired direction.
Front Derailleur: The front derailleur is responsible for shifting the chain between the chainrings attached to the crankset. The front shifter controls the movement of the front derailleur. However, the shifting direction is opposite to that of the rear derailleur. When you shift to a higher gear (harder gear) with the front shifter, the front derailleur moves the chain to a larger chainring. Conversely, when you shift to a lower gear (easier gear) with the front shifter, the front derailleur moves the chain to a smaller chainring. This opposite shifting direction is due to the routing of the cable and the design of the front derailleur mechanism.
The reason for the opposite shifting directions lies in the way the chain moves across the sprockets and chainrings. In order to maintain a consistent chain line and reduce the risk of the chain dropping off the chainrings, the front derailleur needs to move the chain in a different direction compared to the rear derailleur.
While the opposite shifting directions for front and rear shifters may initially seem counterintuitive, it becomes intuitive with practice as cyclists become familiar with the different shifting patterns for each derailleur system.