Asalamu wa alaikum,
Allah says in surat Al Bakarah Ayah 143:
كَذَٲلِكَ جَعَلۡنَـٰكُمۡ أُمَّةً۬ وَسَطً۬ا لِّتَڪُونُواْ شُہَدَآءَ عَلَى ٱلنَّاسِ وَيَكُونَ ٱلرَّسُولُ عَلَيۡكُمۡ شَهِيدً۬اۗ وَمَا جَعَلۡنَا ٱلۡقِبۡلَةَ ٱلَّتِى كُنتَ عَلَيۡہَآ إِلَّا لِنَعۡلَمَ مَن يَتَّبِعُ ٱلرَّسُولَ مِمَّن يَنقَلِبُ عَلَىٰ عَقِبَيۡهِۚ وَإِن كَانَتۡ لَكَبِيرَةً إِلَّا عَلَى ٱلَّذِينَ هَدَى ٱللَّهُۗ وَمَا كَانَ ٱللَّهُ لِيُضِيعَ إِيمَـٰنَكُمۡۚ إِنَّ ٱللَّهَ بِٱلنَّاسِ لَرَءُوفٌ۬ رَّحِيمٌ۬ (١٤٣)
Thus have We made of you an Ummah justly balanced That ye might be witnesses over the nations and the Messenger a witness over yourselves; and We appointed the Qiblah to which thou wast used, only to test those who followed the Messenger from those who would turn on their heels (from the faith). Indeed it was (a change) momentous except to those guided by Allah. And never would Allah make your faith of no effect. For Allah is to all people most surely full of kindness, Most Merciful. (143)
The reason why we face the Qiblah is, in the days the Prophet Muhammad peace be upon Him came, the Qiblah then was towards Jami' Al Aqsa , so Allah change it to the Kabah to see who would follow and who will not.
قَدۡ نَرَىٰ تَقَلُّبَ وَجۡهِكَ فِى ٱلسَّمَآءِۖ فَلَنُوَلِّيَنَّكَ قِبۡلَةً۬ تَرۡضَٮٰهَاۚ فَوَلِّ وَجۡهَكَ شَطۡرَ ٱلۡمَسۡجِدِ ٱلۡحَرَامِۚ وَحَيۡثُ مَا كُنتُمۡ فَوَلُّواْ وُجُوهَكُمۡ شَطۡرَهُ ۥۗ وَإِنَّ ٱلَّذِينَ أُوتُواْ ٱلۡكِتَـٰبَ لَيَعۡلَمُونَ أَنَّهُ ٱلۡحَقُّ مِن رَّبِّهِمۡۗ وَمَا ٱللَّهُ بِغَـٰفِلٍ عَمَّا يَعۡمَلُونَ (١٤٤)
We see the turning of thy face (for guidance) to the heavens; now shall We turn thee to a Qiblah that shall please thee. Turn then thy face in the direction of the Sacred Mosque; wherever ye are turn your faces in that direction. The people of the book know well that that is the truth from their Lord nor is Allah unmindful of what they do. (144)
Facing the Ka'bah is a symbol of unity, and the Ka'bah and what is around it is blessed.
