Ask is a tree and a little box made of tree in sweden

why is ask a word for a tree in swedish?

It's just a typical example of a language barrier. All sorts of words in one language mean all sorts of things in other languages. For example, sex in English means intercourse or gender, where as in German it's the number six. For example, "no" in Portuguese translates to "in" in English. "Sex" in English means "intercourse," "sex" in Swedish means "six." "Die" in English is "death," "die" in German is "the." Like ninety percent of the time it's purely coincidental.