Medieval people's attitude toward wilderness was like of Americans in the last couple of centuries: first the wilderness is a frightening, useless place that must be overcome, and then, when it almost is overcome, it becomes beautiful and valuable and worth preserving.
In the early Middle Ages (sixth-seventh centuries), with the breakdown of Roman communication and trade and indeed urban society, due to plague, the mini ice age, and the rise of Islam, Europe became predominantly rural and heavily wooded. It took several centuries before the population began to grow enough that people became aggressive about clearing land for crops.
The wild wood was frightening, a place for wolves and bears (still found in Europe then), a place all too easy to get lost in. Villages were far between, with nothing like the road-signs or even roads we now take for granted. People lived in the woods, of course, bandits or half-crazed hermits, but they were just as scary as the wolves and bears.
In the eleventh century, with a growing population and the new, heavy carruca plow, which allowed plowing of denser, damper, and thus richer soils, there was a concerted attack on the wilderness. Marshes were drained, brambles cleared, trees chopped down to create new fields. Both peasants and monks tackled this enthusiastically--monks of the new Cistercian order depicted themselves at the beginning of the twelfth century as energetically cutting down trees. Landlords too set out to turn waste land into agricultural land, to increase their income.
The same response to what must have seemed like infinite woods motivated settlers in the American colonies. The whole eastern half of what is now the US was heavily forested when white men arrived. As late as around 1800 in Ohio, centuries-old oaks were being chopped down so that new corn fields could be planted, the trees being burned in enormous bonfires that could be seen twenty miles away.
But in medieval Europe, as in the Americas, people quickly discovered that the forests were not infinite after all. It was considered a miracle when the abbot of Saint-Denis (near Paris), early in the twelfth century, was able to find enough large trees to make the beams for the new church he was building.
Wood was necessary for all sorts of things, from building houses (as well as churches), to building ships, to making carts and furniture, from making trellises for vines to making the cogs for windmills. Metal was scarcer than it is now, and plastic was centuries in the future. And especially wood was the chief fuel in an age without oil, gas, or coal. Forests were also places for wild animals that could be hunted, for pigs to be pastured, and for wild honey and berries to be harvested.
Twelfth-century Europe probably had fewer woodsy patches than it does now. For the first time, people started worrying about indiscriminately chopping down forests. The problem was especially acute in Iceland, an island far enough out from the Continent that tree seeds do not blow there readily, and birds do not carry acorns across the waves. When the Vikings first settled, it was wooded, but they cut down the trees, for their ships, for fuel, and for pasturage for their sheep, and the trees did not come back. Even now, after many years of planting trees, Iceland has a fairly barren landscape.
Those woods that remained on the Continent continued to be homes of hermits, though from some of the accounts there seem to have been quite a large number of hermits squeezed together in the few remaining patches that could be considered wilderness.