Welcome to Gaia! ::

http://www.dreammoods.com/
I find is fairly acurate to cover the basics

Phoenix symbolizes immortality and renewal.
Lake signifies your emotional state of mind. If the lake is clear and calm, then it symbolize your inner peace.
Reflection represents your true self; it is time to look within. The reflection may highlight both your flaws and positive attributes. Learn from your flaws and how to improve on them and at the same time appreciate your good qualities. Alternatively, your reflection could also indicate how you want others to perceive you.
Pluto indicate a rebirth or transformation.
Blue and purple: tranquility and cleansing

Such a cool theme and is entwined so well smile

If I'm not mistaken the other pheonix may be your current Spirit Guide. All the inner work you have been doing is paying off, and you are undergoing a transformation (renewal, rebirth) and have just been given a glimps of your achievement.

Not sure
If the sadness isn't directed at something particular in the dream then it could be something in real life that you are feeling a bit down about, but may not recognizing it or letting it be recognized (denial or hiding true feelings).

If that is the case, maybe that is the transformation that needs to take place in order to be like your pheonix

I concur with Thyna (I love that site by the way!), but I believe that not only is the Pheonix your Spirit Guide, but the reflection is YOU..on the inside. That may be why you day dream about her.

And Oterys theory seems to be a good one as well.