The seeds on rocky ground are those who hear the word and receive it with joy, but they have no root. They believe for a season, but in the time of testing, they fall away.
The former paragraph began with an account of Christ's industry in preaching (Luk 8:1); this begins with an account of the people's industry in hearing, Luk 8:4. He went into every city, to preach; so they, one would think, should have contented themselves to hear him when he came to their own city (we know those that would); but there were those here that came...
Commenting on Luke 8:4-21
And that which fell among thorns are they,.... The seed that fell among thorns, or were sown on thorny ground, represent such hearers: which, when they have heard, go forth; from hearing the word to their worldly business; or go on in the pursuit of their worldly lusts and pleasures notwithstanding; for the word translated, "go forth", belongs to the next clause; and are choked...
Verse 13. (o) "receive the word with joy" (p) "have no root"