Python 3, 85 bytes

Based off @ugoren's Excel answer@ugoren's Excel answer, here's my Python 3 answer that isn't special at all.

for c in range(26):
    for r in range(26):print(chr(65+max(c,r)),end='')
    print()

Explanation:

for c in range(26):     # The grid is 26x26,
    for r in range(26): # so we have to do 2 for loops
                       print(chr(65+max(c,r)),end='') # Some formula to print the letter at coordinates (c, r)
    print() # Print a newline after a row so that we can print the next row