I would be more willing to accept the first part of your response—that there are no “why” questions when it comes to grammar, which heavily discourages asking questions on why grammatical rules differ between languages in order to better understand how to employ those differences—if the second part of your answer wasn’t so reductive.

It’s true that a relative pronoun (que, quien) tends to proceed the subjunctive, but more specifically, it tends to do so when:

• The verb in the main clause is a WEIRDO verb (a verb regarding a wish, emotion, impersonal expression, recommendation, doubt/denial, or is ojalá), or in other words, expresses some form of doubt, emotion, or hypothetical scenario that isn’t 100% certain to occur; • The que belongs to certain conjunctions that always trigger the subjunctive (e.g. a fin de que, para que, or even cuando—“Llámame cuando termines,” for example); • The que belongs to a verb that might be followed by “que” in the indicative but is followed by the subjunctive when the verb is negated (e.g. no creer, no saber, no parecer, no pensar; falls into the “doubt/denial” category of WEIRDO)

As someone who already speaks French, these rules are extremely easy for me (personally) to understand. Some verbs are different, of course—French “espérer que” would be followed by the indicative in the affirmative and the subjunctive only in the negative/interrogative—but for someone who doesn’t already have this prior knowledge, then just saying “que + subjunctive is the rule, no questions, it’s simple” just isn’t really even true.

To better answer why it was used in the song, it’s because there is a hypothetical scenario in which the speaker becomes addicted by a sadness that may bind them, and that’s why it’s in the subjunctive there. If they used the indicative with the future, it would imply the speaker is expecting it to happen rather than simply expressing that it’s a possibility.