But we, brothers, being bereaved of you for a short season, in presence, not in heart, tried even harder to see your face with great desire,
Paul is referring to the mob violence that precipitates his departure from Thessalonica. He has no desire to leave those babes in Christ bereft of a spiritual father. He would have extended his stay among them until they are fully instructed in the way of the Lord but for the threats against his personal safety. He describes his absence as "a short time," but it very likely lasts some five years.