I don't know -- as per the chart below, Tocharian is as related to GErmanic, hittite, Indic, Irani languages as it is to Celtic -- it's just another branch of the Indo-European langauge family
Can't argue...I'm just quoting Victor Mair and Elizabeth Barber.
Cronos,
Charts are, afterall, just a representation of scholars' best conjecture. Some past linquists have declared Tocharian to be a Celtic language based on word and grammar similarities between Italic and Celtic. I believe most linguists today don't go quite so far, but they do associate the word and grammar similarities more closely with European language groups (Greek, Armenian, Baltic among others).
Remember, there is no existing language today called Proto-Indo-European, and there are no written records of that language. It has been reconstructed by examining word similarities between related languages. The closer the match, the later the languages split from one another. Tocharian A and B show a very ancient relationship with European languages and later influence by Indic languages.
It is still not a settled question and may never be, but it is interesting none the less.