12th house is house of Spirituality. 12th sign Pisces is sign of Spirituality. 12th house is final house and 12th sign is final sign.

It means that Spirituality should be our final or last pursuit. It can't be our 1st pursuit of life. 1st we need to exhaust all our materialistic desires and then only we should move towards spirituality.

But if someone is born as an evolved soul and doesn't have any materialistic desire then he can directly move towards spirituality. That soul had already exhausted it's materialistic desires in past lives.

12th house is basically a House of Losses. So, whichever planet is there in 12th house, we begin the life with feeling a sense of loss related with that planet. That planet improves during the life when we give our best efforts for that planet. In other words, consider that planet in the bucket of losses to start your life and now if you want to gain anything related with that planet, you may need to put extra effort throughout life. Like,

Sun - person will have lack of confidence or self-worth. 

Moon - lack of focused mindset, too imaginative. 

Mars - lack of courage & will-power, kind of lazy. 

Mercury - lack of ability to communicate correctly, feeling of loss towards relation with siblings. 

Venus - feeling of loss in relationship, lack of idea as to how relationship can be improved. 

Jupiter - possibly loss of education or morals. 

Saturn - loss of sense of duty or responsibility in life, again sort of laziness.

So, person can have or at least feel a loss in these areas "to start with" in life but as he grows up and works on these planets, he can get better results in later age.