快: why does "fast" also mean "happy"?
Not a very satisfying answer but anyway:

說文解字


  喜也。从心夬聲。苦夬切


說文解字注


  喜也。引申之義爲疾速。俗字作駃。从心。夬聲。苦夬切。十五部。


Notice the 引申之義爲疾速 from above, happiness extended to mean fast.

I don't know exactly the mentality of 喜 -&gt; 引申 -&gt; 疾速 but apparently people did think they were related at one point in time.

Maybe it has to do with 俗字作駃

大汉语词典


  駃 kuài［1］快马。“参见駃马”。［2］同“快”。


Must have been some mix-up somewhere.