Why does Allah test us???

It was narrated by Abu Huraira (may God be pleased with him) that God's Messenger (peace be upon him) said:

"If God wants to do good to somebody, He afflicts him with trials." (Sahih Bukhari Volume 7, Book 70, Number 548)

It was also narrated by Abdullah (may God be pleased with him):

“I visited the Prophet during his illness and touched him while he was having a fever. I said to him, "You have a high fever; is it because you will get a double reward?" He said, "Yes. No Muslim is afflicted with any harm, but that his sins will be annulled as the leaves of a tree fall down."” (Sahih Bukhari Volume 7, Book 70, Number 565)

In chapter Al-Baqarah (The Cow), verse 155 we also find:

{Be sure we shall test you with something of fear and hunger, some loss in goods or lives or the fruits (of your toil), but give glad tidings to those who patiently persevere} (Quran 2:155)

Also, in chapter Muhammad, verse 31 we find:

{And We shall try you until We test those among you who strive their utmost and persevere in patience; and We shall try your reported (mettle).} (Quran 47:31)

Finally, in chapter Al-Ankaboot (The Spider), verse 2 we find:

{Does mankind think that they will be left to say, "We believe", and that they will not be tested?} (Quran 29:2)

And one of the reasons Allah (SWT) says is so that they will come back to Him:

“And We have sundered them in the earth as (separate) nations. Some of them are righteous, and some far from that. And We have tried them with good things and evil things that haply they might return.” (Al-A’raf 7; 168)

Allah is actually giving the test so people will come back to Him and raise their hands and ask for help.

In another ayah Allah (SWT) says another reason for hardships, tadaru', which is a very strong concept that could be translated as: humility or humbling yourself:

“We have sent already unto peoples that were before thee, and We visited them with tribulation and adversity, in order that they might grow humble.” (Al-An ‘am 6; 42)