This is what makes milk such a rich metaphor and such a rich dream symbol ripe for interpretation.
If you have a dream in which you need to drink milk it means that you feel close to your mother right now. Drinking milk nearly always has to do with motherhood or needing to be near your mother in some way when it comes to dreams. Sometimes it may have something to do with your recent contact with milk if this is something that has happened to you recently but most probably you have been thinking about your mom lately which is what has lead you to thinking about milk and which has in turn lead to you seeing yourself drinking milk in your dreams. This is the best explanation and the next time you see your mom be sure to say hello and be happy that you know her. Many people out there are not as lucky.
If you have a dream that you are drinking milk then this could simply mean that you are in need of that same comfort that milk gives to so many others. You feel a little left out and you want to feel not only more included but more loved. You need to feel at peace with what is around you and at peace with life. This can be done any number of ways but only you can answer the question of what makes you personally more at peace with yourself. Seek out what this is in the real world and absorb its peaceful energy.