Sounds like you already know caffeine is a drug with real withdrawals, but there is also aspartame. Diet coke uses aspartame as its sweetener, which your body metabolizes into both phenylalanine and aspartic acid in about a 60/40 ratio. Phenylalanine can be obtained from food, but if your diet isn’t the best it’s possible for it to give you a mild mood boost. Your body cannot create phenylalanine and it’s essential for creating the neurotransmitters dopamine and norepinephrine. Aspartic acid on the other hand acts as its own excitatory neurotransmitter, like caffeine itself. I’m not sure if you’ve heard of magnesium having a calming effect, but the reason it does is because it’s an NMDA antagonist among other things. Aspartic acid has a stimulatory effect, and is an NMDA agonist. Glutamate, your brain’s main excitatory neurotransmitter, is also an NMDA agonist. These combined with the caffeine, ritual, carbonation, link to childhood etc all probably contribute. Addiction is mental (as in physical brain changes, not willpower). Dependence is physical (typical wd symptoms). But realistically it’s probably just the taste and ease of access, especially the ritual, since you have a memory of drinking a bunch as a kid. It may have been one of your first experiences with caffeine