Musar for Kiddushin 58:14
ת"ר ללמוד תורה ולישא אשה ילמוד תורה ואח"כ ישא אשה ואם א"א לו בלא אשה ישא אשה ואח"כ ילמוד תורה אמר רב יהודה אמר שמואל הלכה נושא אשה ואח"כ ילמוד תורה
perhaps a miracle will happen [in his merit].' So he [R'Aha B'Jacob] entered and spent the night in that schoolhouse, during which it [the demon] appeared to him in the guise of a seven-headed dragon. Every time he [the Rabbi] fell on his knees [in prayer] one head fell off.
Shenei Luchot HaBerit
The usual answer given is that Rashi wanted to explain why Isaac had not married earlier. Our sages say that if someone is not married by the time he is twenty years old G–d makes such a person's bones swell up (Kidushin 29). Rashi therefore gives an accounting of all these years to show that Isaac had not been able to marry a suitable wife sooner.
Ask RabbiBookmarkShareCopy