We forget about our past lives because our souls are recreated and reformed by the creator. Depends on what you follow (religion) it brings upon a different ideology and reasoing.

I agree with Susanna. At birth and younger years it is more clear as to who we are in other realities.. For the current incarnation, the whole idea in my perspective is to embody the 3 dimentional reality to carry out the current incarnation. Hence , our focus narrows to incompass this existance.
I have clear memory of being born "here" in this time frame, with the knowing of who I am. With the awareness of those arround me, be it in physical or etherical form. I have always known "since birth" I am not of this world (reality,dimention,) I have always known this birth into 3 dimentional is just one experience, a fragment of who I am.
This does not mean I have not forgotten while living this present existance who Iam.. trust me, there have been periods in this present existance where I lost contact with my knowings.. Yet all for the good.. of the experience.
I also agree with the perspective of vibrational energy and freqencey . The 3 dimentional reality which we all at present moment exist in, has a slower rate of vibration. It is my perspective that we can alture this freqencey of vibration to see and hear and feel different  knowledge and information.
If anyone has any thing further to add feel free.