Question

how are lines form in our hands

Answer 1

Scientifically speaking,the lines on a palm or palmar flexion creases,help the hand's skin stretch and squeeze.
They also help identify certain medical conditions.
Human hands develop palmar creases in the womb at around the 12th week of gestation and are present on the baby's when he/she is born.

Answer 2

Hand is a mirror of our personality. Its size, shape, texture, skin pattern, finger prints, length of fingers and Mounts of our hand reveal our basic tendencies and make us aware of our own personality, potentials and talents. All these features along with lines make our future. After knowing and understanding these features we can know hidden capacity of the person.
Lines of our hand are not the creases on hand but they represent the activity of our unconscious mind (Hindus call it MANN).
More lines in our hand means more active mind. According to the research of Prof. Dayanand (Head of the Institute of Palmistry) the Mounts on our hand work as magnetic centers which attract electric currents of brain and lines on the palm show the path of these currents. Reading the lines is the reading the hidden capacity of a person.