There are some theories that intelligent alien life might be more humanlike than you would expect considering the variety of conditions in the universe. Chemistry works differently at extreme temperatures in either direction, so even if you skip over the idea of carbon-based life you still hit some limits with what sorts of complex chemistry can happen with extremely heavy or extremely light elements.

If you decide life can still exist in those conditions and start seeking increasingly exotic forms of life that we can’t even comprehend there are some other limits to consider. Aliens that are too large or too small are much less likely to ever become a space-faring species simply due to the physical constraints of building a machine that can escape orbit, even when considering very non-earth-like planets. Certainly traveling through space isn’t required for intelligence, but if we’re ever going to encounter life it’s much more likely that we'd encounter a space-faring species.

Then when you look at the shape of humans, we are symmetrical which has the benefit of allowing binocular vision that gives us a better grasp of 3d space, we have limbs that allow us to move around our environment and manipulate objects to build tools. Certainly there are other ways to accomplish these things, but these traits are simple and shared amongst a lot of life on earth because they are good solutions to a broad variety of environmental challenges.

Basically just consider the things that an intelligent space-faring species would need to be capable of and work backwards to the type of physical characteristics they would need to have and then the type of environment that would allow those traits to evolve. Alternatives are certainly possible but we definitely don’t know what else to look for.