"Torah learning does not endure for one who is lax about it, one who is too leisurely, or one who learns while eating and drinking; only for one who 'kills himself' over it and agonizes over it, withholding sleep from his eyes..." (שו"ע סכ"א ). This means that only one who "kills himself" over his Torah will not forget it (שו"ע הרב פ"ד ס"ט ), for the only wisdom that endures is that learned through toil and labor; when learned pleasurably and leisurely, it will not last or be of benefit (פיה"מ אבות פ"ה מ כ "ג ).
The main aspect of toil in Torah is that of pilpul and the back-and-forth in order to truly understand it, as it says, "im bechukosai teileichu" – that you should labor in Torah. This explains the text of the brocho, "la'asok bedivrei torah" – Torah should be like an occupation with "negotiation" (ט"ז או"ח סי' מ"ז סק"א ).
