Today I went to paddle with a friend and he said that I should sit in the back because I'm heavier. But why is that?
How does this help with the weight distribution or handling of the kayak?
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It's along the lines of @stijn's comment but worse: if the nose digs in, not only do you lose speed, but your control suffers (and not just because many steering strokes require water speed). In even slightly choppy water the bow tends to dig in just when you don't want it to - when you want control.
Partly because of this, the front seat tends to be quite small, so you may not be able to put a tall person in there.
A bigger person can also apply more steering force - they're likely to be stronger as well as having more leverage. This makes stern rudders more effective, among other strokes. These are long boats and take some steering.
A further factor is that experienced or supervising paddlers are often larger and stronger with better technique, because novices are often children and even adults new to kayaking won't have trained their kayaking muscles. The occupant of the rear seat can keep an eye on the occupant of the front seat. It's also possible to roll a whitewater tandem kayak from the back alone - I've done that in a topo duo after the front paddler bailed, and also with a passive front paddler (as a training exercise or challenge).
Look at open canoes with one paddler. The paddler is either central or well back. Last time I was in a canoe I was in the middle with my young daughter at the front. For control and trim I'd have been better further back (especially as it was a very wide boat) but I wanted to be closer to her as it was her first time in a boat. It turned out she was too small to paddle so didn't need any coaching, and I could have sat/knelt at the back.