Why does my girlfriend love onions, but hate garlic?
While onions, garlic and leeks are related, their flavours are not the same and a person may well like one but not another, or even like one cooked one way but not another (for example, fried but not raw onion, or roasted but not raw garlic). You ask &quot;chemically, what's going on here&quot; – there are different chemicals, as you can surely intuit by tasting the three for yourself (and each chemical will change during a given cooking process too).
Food preferences are idiosyncratic, and while sometimes you can identify a reason (for example, there is a specific gene related to finding coriander/cilandro unpleasant, or someone might have previously had food poisoning after eating a particular item and afterwards be unable to stomach it) on the whole you shouldn't expect an explanation deeper than 'because she does'.