A material worldview would say that the physical chemistry creates an emotional affect where "we" feel this "falling in love". Yet, it can't work the other way around with "falling in love" affecting the brain unless one accepts mental states aren't all derived from physical states.
There are several versions of materialism but the version I go for is emergent properties and supervenience. (see https://en.wikipedia.org/wiki/Supervenience
) A feeling in the brain consists of a pattern of neurological states that consists of a number cellular states, of moleculars, of atoms that consist of .... Each level of description supervenes on a lower level. That means that if there is a specific feeling that consists of for instance a pattern of molecules, then if this pattern occurs again, this feeling occurs, i.e. the feeling is associated with the molecule pattern. If you just look at the molecule level if will extremely complicated to describe what is going on but if you go to a higher level, feelings emerge and you may be able to understand what's going on.
Another example, waves of water consist of water molecules. When describing systems of waves it is in principle possible to do that on the molecule level but it much much simpler to do it on the wave level. Waves are an emerging property that supervenience on the molecules.
This is intended to explain how "falling in love" is both a high level property as well as a state of brain molecules and that therefore there is no problem how "falling in love" can affect other parts of the brain.
Saying that the brain is only "chemical reactions" is almost as clever as saying that the Watson computer winning Jeopardy is just some silicon, only a million times worse.