Maarif Ul Ahwal معارف الاحوال

There is another explanation for not making Khushu خشوع ` a necessary condition for the total duration of the prayers.
In certain other verses, the Holy Quran has clearly enunciated the principle which governs legislation in religious matters: nothing is made obligatory for men that should be beyond their endurance and power.
Now, except for a few gifted individuals, men in general are incapable of maintaining Khushu خشوع ` for the total duration of the prayers; so, in order to avoid compelling men to a task they cannot accomplish, the Fuqaha have made Khushu خشوع ` a necessary condition only for the beginning of the prayers, and not for the whole duration.In concluding the discussion, Imam al-Ghazali (رح) remarks that in spite of the great importance of Khushu خشوع ` one can depend on the infinite mercy of Allah, and hope that the man who offers his prayers unmindful will not be counted among those who give up the prayers altogether, for he has tried to fulfill the obligation, has turned his away from everything to concentrate his attention on Allah even for a few moments, and has been mindful of Allah alone at least while forming his intention for the prayers.
Offering ones prayers in this half-hearted manner has, to say the least, the merit of keeping ones name excluded from the list of those who habitually disobey Allah and forsake the prescribed prayers altogether.In short, this is a matter in which hope and fear both are involved - there is the fear of having incurred punishment as well as the hope of being ultimately forgiven.
So, one should try ones best to get rid of ones laziness and indifference.
But it is the mercy of Allah alone which can help one to succeed in this effort.