Why is it theorized that our sun will become a red giant?
The evolution of stars is determined by their mass, and yellow dwarf stars like our sun are massive enough to eventually fuse helium in their cores.
Every star, regardless of mass, starts out as a main sequence star. Main sequence stars fuse hydrogen into helium. Eventually the helium builds up in the core, and the rate of fusion slows down. Without the energy from fusion, the core begins to shrink and heat up.
For red dwarf stars, which are less massive than the sun, the core doesn't heat up enough to trigger helium fusion, so a red dwarf will remain a red dwarf until it cools into a white dwarf and eventually a black dwarf, which is a hypothetical star phase, due to the fact that the amount of time passed isn't sufficient yet for anything more than the white dwarf stage.
When a yellow dwarf star like our sun runs out of hydrogen and the core contracts, the temperature gets hot enough to fuse helium. This increased temperature causes the outer layers of the star to expand. As the star gets larger, the surface cools and becomes redder. At this point the star has become a red giant.