hashkafah philosophy - How come Bais Hillel is usually lenient and Bais Shammai usually stringent?

It seems like the Bais Hillel had a tendency to be lenient in Halachic matters vs the Bais Shammai, it was a sort of a presumption to them in which they were almost always successful to back up with their methods of Derash (and the same goes for Bais Shammai regarding their stringency). Was it a principle the Bais Hillel believed in (perhaps it was based on "Deruchehu Darchei Noam")? or was it some kind of train of thought? We find the same pattern in the later Achronim. For example: R moshe Feinstein vs. Chazon Ish...