In an alternate timeline where Mars is the second Earth-like planet in our Solar System, the drive for colonizing it would perhaps have been much stronger. There would've been more resources spent exploring it and studying its countless species completety unrelated to life on Earth. The first manned mission might even have been launched right before the dawn of the 21st century! But for that to happen, there needs to be some prerequisites, prerequisites that strech all the way back to 4.5 billion years ago, back when the planet was formed.
Mars resides in the outer reaches of the habitable zone around the Sun. If it had an atmosphere like Earth, with the same greenhouse effect like Earth, it would've been just another colder, icier version of our planet, with a more turbulent global climate cycle (due to it not having a large moon to moderate its axial wobbling). The reason why it's arid and barren today, is because it lost its magnetic field, allowing the solar wind to blow away most of its atmosphere. Atmospheric loss was rapid because Mars only has 10.7% the mass of the Earth. The low mass might also be why the magnetic field decayed so early. Without sufficient atmospheric pressure, liquid water would boil away, split into hydrogen and oxygen, with the oxygen getting absorbed into minerals on the ground and the hydrogen blown off into space by the solar wind.
But what if, Mars formed with more mass, especially with enough iron to retain its magnetic field until today, or at least enough mass to hold onto a thicker atmosphere till today? How much more mass does it need from the start in order to be another Earth-like planet today, ready for human exploration?