Just an intuitive feeling that the surface of stars is extremely hot but under the surface all types of different atmospheres could exist and so the motion of energy could move through a star like xyz rivers, leaving us with a possibility of life conditions inside the star.
Our primary method of understanding stars is using optical telescopes. We can directly measure the temperature of the "surface" of a star by fitting its spectrum to a blackbody. However, this gives us the temperature only at the "surface" or the optical depth of a visible photon.
We know that stars are extremely hot through the hydrodynamic/thermodynamic modeling. If the surface is hot, that heat must have come from somewhere. The only place it can come from is from the star's interior.
We know that stars, like our Sun, are fusing hydrogen into helium. This is because the mass and volume of the Sun, would give it central pressures and densities that would be hot enough to do so.
We can also directly measure the resulting particle byproducts of this nuclear fusion (a very hot process) by using neutrino detectors.
Unfortunately, life does not exist within the Sun because the temperatures are millions of Kelvins. These temperatures mean nothing more complicated than atomic nuclei and free electrons exist at the very center of the Sun.