why does McDonald ask Holmes for help

From the text:

Twice already in his career had Holmes helped him to attain success, his own sole reward being the intellectual joy of the problem. For this reason the affection and respect of the Scotchman for his amateur colleague were profound, and he showed them by the frankness with which he consulted Holmes in every difficulty. Mediocrity knows nothing higher than itself; but talent instantly recognizes genius, and MacDonald had talent enough for his profession to enable him to perceive that there was no humiliation in seeking the assistance of one who already stood alone in Europe, both in his gifts and in his experience. Holmes was not prone to friendship, but he was tolerant of the big Scotchman, and smiled at the sight of him.