Shiva represents consciousness, the divine consciousness or the Self and Shakthi represents vibrant energy, vitality, the creative power. Shiva and Shakti represent the masculine and feminine aspects on all levels of existence.The self-complementary Ardhanareeswara (half man, half woman) concept. When this androgynous, self-complementary energy is divided into two, they turn into the latent or passive consciousness and the pure, vibrant energy without any consciousness. OR we can say that, Shiva is the passive or latent Shakthi and Shakthi is the active Shiva. Shakthi or the active Shiva can unleash untold miseries. We have heard the story of Kali (MahaKali) and her unbridled fury which was tamed by stepping on Shiva. This is a symbolic representation of how the divine consciousness neutralizes the violent, mindless, raging energy.
Let us come back to the topic of discussion - Why is the active energy of the Universe depicted as female? So far nobody knows who did or when this division and naming occurred. The only guess one can make is that highly intelligent and developed creatures have walked this planet, at times beyond our imagination. So let us try to explain this from our level of understanding.
Let us look at it from the cellular level. Our cells contain free floating organelles called mitochondria. They are known as the powerhouses of the cell. They function like a digestive system of the cell. They take nutrients, breaks it down and create energy rich molecules for the cell. This bio-chemical process is known as cellular respiration. Mitochondria contains their own genomes. The human zygote contains only the mitochondrial DNA from the egg cell, the sperm mitochondrial DNA do not contribute genetic information to the embryo.So the powerhouse of the cell or the mitochondria is inherited from Mothers. At a cellular level itself energy source is contributed by the females. This might be a reason why energy is considered female.
In each person, the Shiva and the Shakthi are there, but are set apart. Shiva or the divine consciousness resides in the Sahasrara Chakra and Shakthi or the divine creative energy is coiled up in the Mooladhara Chakra. So each person is incomplete as such. So, to attain perfection or moksha, in each life, a little effort and time should be spend in bringing the Kundalini energy coiled up in the mooladhara to the Sahasrara. Otherwise one will be tied up in samsara endlessly.