Too much dopamine is not a good thing. Adding theanine to compounds that boost dopamine like caffeine, can protect against the effects of excessive dopamine.
Adding theanine to stimulant products is a great way to smooth out the effects and make them much more useful and usable if you will. Rather than getting too "tweaked" out and overly stimulated.
This is a big reason I like to include theanine in combination with high dosed yohimbine and moderate caffeine as a fat loss stack.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5018574/