The speaker compares relationships to candy, and the comparison starts out light and casual. "I like to think" sounds offhand, as if this is a private way of making sense of love. Some relationships, like some candies, are interchangeable: they look alike, taste alike, or simply vanish too fast to matter.
The second half turns more serious. The jaw-breakers stand for bigger, longer relationships, and the detail that they "numb you with each lick" suggests something that doesn't just sweeten life but dulls the speaker's feelings. They last longer, but the speaker isn't quite enjoying them. Then comes the sting: they "don't last forever / like the package says they will." That line reads as a feeling of being let down, even misled by a promise of permanence.
The mood is wry and a little bitter, but quiet rather than angry. The simple, childlike image makes the disappointment feel familiar, almost resigned, as though the speaker has learned this lesson more than once.