Why do we expect life mostly on Mars?

There had been strange beliefs that life existed on Mars and Moon. But the concept of life on Moon was discarded when the face on the moon(many craters on its surface looked like a face) was proved to be only the craters. But the search for Extra Terrestrial Life continued, and people thought that there was life form on Mars.
 And the main reasons for that are:

Photo of microscopic rock formations
indicating past signs of water

1. Misunderstanding: In 1877, an Italian Astronomer Giovanni Schiparelli saw a series on Mars. He called them canali, the Italian word for channels. This word was translated into English as 'canals', by mistake. Thus many people thought that intelligent life on Mars, dug these canals.
   
2. Discovery of water(indirectly):  The rocks that were found on Mars' surface, showed signs of water. The photo alongside shows the formation of a microscopic rock which is possible only with the presence of water.
   
3. Channels: Viewed from the telescope, Mars shows lines on its surface. These lines must probably be channels in which water would flow, and also to become permanent, it has to be water.
   
4. Discovery of Hydrocarbons: Hydrocarbons like Methane and Formaldehyde were detected on Mars recently by the satellites on Mars and hint that life could have existed in it. Methane gas is produced when organic matter(like life) decomposes.