Why are there significant changes in the body at puberty stage

Because the organism needs to become able to reproduce.

Explanation:

Puberty is defined as a physiological process of hormonal maturation and growth that causes changes in an individual's body making the organism able to reproduce and coincide with adolescence. During puberty there are changes in the pattern of secretion of some hormones, predominantly testosterone in boys and estradiol in girls. With the exception of the fetal period, there is no other stage in human development in which growth in height and changes in body composition are so intense and rapid. The pubertal stretch lasts about three to four years and represents a gain of approximately 20% of height and 50% of the individual's adult weight. Adolescents of the same age are often at different stages of puberty, as puberty has a very variable onset and progression rate between them.

her affection for her parents makes her an especially sympathetic character.