And now, as he saw it, Grim stood committed to a course that could only lead to trickery.

And all trickery must pave the way for plunder.

And plundering was fun.
His sons and grandsons in varying degree saw matters from the old man's viewpoint, although, having had rather less experience of it, they were not quite so confident of Grim's generalship; but they made up for that by perfectly dog-like devotion to "the old man, their father," whose word and whose interpretation of the Koran was the only law they knew.
What tickled their fancy most was Ali Baba's cleverness in egging on Ayisha to advertise Grim as Ali Higg.

Again and again on the march that day, in spite of the grilling heat, and thirst and flies, they burst into roars of laughter over it, chaffing Ayisha's four men unmercifully.
And after a while Mahommed, the youngest of Ali Baba's sons, regarded by all the others as the poet of the band and therefore the least responsible and most to be humored in his whims, made up a song about it all.

It called for something more than boisterous spirits; it needed the fire of enthusiasm and ingrained pluck to set them all singing behind him in despite of the desert heat and the dazzling, bleak, unwatered view.