Why do people touch wood for luck?

An old superstition

Not overly helpful.

However, the superstition of touching wood (or iron) as a counter to evil goes back centuries. There are various thought on the origin.

One says it comes from the touching of wood from Christ's cross as a protective talisman.

Another pre-dates Christianity in that both the wood of the oak and the ash trees were sacred to ancient pagans.

The touching iron comes from the old belief that iron will keep the "fairy folk" away from you, as iron will kill the fae.

There is also the "knock on wood" variation, which was originally done to wake the dryad who lived within the tree (wood).