Rashi: What you thought about during the day ? what will be after you ? came [in a dream] on your bed. Orchos Tzadikim (Sha'ar ha?Emes) - dreams are via an angel that accompanies man. Since all his thoughts are not Emes, not all his dreams are Emes. One who accustoms himself that all his thoughts are Emes, also at night he sees true visions, and he knows the future, like angels.

Malbim: Because you prepared yourself, via desire to know what will be after you, Hashem informed you this, and also what will be until the end of days.