Wishes are thorns, he told himself sharply. They do us no good, just stick into our skin and hurt us."

The line shown in the question above is an example of a metaphor.

Explanation:

The metaphor is a figure of speech that establishes a subjective comparison between two elements that have no explicit relation, but that can be compared in a poetic and implicit way creating a new meaning. Unlike the Simile, the metaphor does not always use the words "like" or "as" to establish the comparison. The sentence shown in the question above is an example of this, where wishes and thorns were compared implicitly, without using the words "like" and "as".

because they were not just any seashells they were sally's seashells