Can anyone interpret a dream?

Dreams can have many interpretations. Since it was your dream, your interpretation is probably the best. There are many methods at getting to a meaning of a dream. One method is through a psychological system called Gestalt. In this system, the method is to get 2 chairs. Set the chairs so they are facing each other. Sit in one chair. Chose something in the dream that stands out - such as the milk carton in your dream. Imagine the milk carton is 'sitting' in the chair opposite you. Ask the milk carton a question, any question. Then physically move and sit in the 'milk carton' chair and answer the question. Just say whatever comes up for you, no judgement even though it may seem stupid or foolish. After answering, move back into your chair and ask another question. In this manner, have a conversation with the milk carton. It may seem stupid but after a couple of iterations, some interesting information comes out. There are psychologists that you could go to that will mediate that process for you if you want. Also, for all dreams, it is good to write them down, in detail, as soon as you wake. There are also many websites that talk about dreams and symbolism.

Really Will, the answer is in you head. If milk symbolises fertility, then being upset on your girlfriend for some milk left outside, may actually mean that you somehow consider her unfit to be the future-mother of you children. Consciously or unconsciously, you may think she's not what you need... If you dream this again, or if you dream her somehow disabled, then you should understand that your subconscious is warning you.

Don't be hard on you girlfriend when she leaves food out.

This dream tells you that it's not a big deal to leave food out, the milk was still good.

So you unconsciously know that it's not a big deal.

Your unconscious has been interpreted into the dream so that you can know it.