Question

Why did the Palestinian Liberation Organization set up bases in southern Lebanon?

Answer 1

The base for Palestinian Liberation Organization were set up in the south because of the following major reasons.
None of the other Arab countries wanted them, they had been driven out of Syria and Kuwait, and while a large number lived in Jordan, they were considered as outsiders by the dominant Hashemites. However Lebanon accepted some of them, although many remained in refugee camps. 
Another reason was, Southern Lebanon was bordering Israel, (much of which had been part of Palestine until 1948; both Jews and Arabs had dispute over which group should own the land). The proximity of Southern Lebanon to the state of Israel enabled the Palestinian Liberation Organization to launch offensives, in an effort to defeat the Israelis and recapture land they believed to be theirs.