What is the reason behind the diagonal relationship of elements of the 2nd period in the modern periodic table?

@ manav 
the diagonal relationship exist due to the fact that by moving from top to bottom and left to right in the periodic table , they both have opposite effect to the trend . 
therefore on moving diagonally the two opposite effect tend to cancell out and the value of the trend will remain almost same . 
thus diagonal pairs have many simmilar property . 
HOPE IT CLEARS 
ALL THE BEST ..