Mars is the fourth planet in the solar system. It lies between the asteroid belt and the earth and thus is the closest celestial body to the earth after the moon. The conditions in mars are pretty much hospitable. That is because, Mars is close enough to the sun to be warmed by it and far enough to be unaffected by its changes in heat output. Its closeness and hospitable conditions make it more accessible to us than most planets in the solar system.
From ages people have been speculating about little green men from Mars intruding earth. What started this? No one knows with conviction. Astronomers saw crisscross lines on the surface of mars and assumed it to be irrigation canals. This could be one reason.
Another reason for scientists to expect life on Mars was that there were apparent seasonal color changes on the planet's surface. This led to believe that conditions might support growth of Martian vegetation during the warmer months and cause plant life to become dormant during colder periods.
No one knows for sure. Maybe that is the reason why the curiosity about this planet is alive till date. Mars may very well be the most explored planet in our solar system. This is based on the contemplation that this planet resembles early earth.