in order to fulfil His plan

When we born, we have basic character and temperament. sanguine,phlegmatic,choleric,or melancholic. But - please notice this - in order to fulfil His plan through us and His wants, actually, we have to chase after having Christ's character in us.
I John 2:6 He that saith he abideth in him ought himself also so to walk, even as he walked.

Matthew 5:48 Be ye therefore perfect, even as your Father which is in heaven is perfect.

for instance, it is ok you are be born as melancholic, but it doesnt mean you will die as melancholic.

To fulfil His great plan in us, we have to be changed. Having His character. Changed from very human into divine life or character.

John 15:1 I am the true vine, and my Father is the husbandman.
15:2 Every branch in me that beareth not fruit he taketh away: and every branch that beareth fruit, he purgeth it, that it may bring forth more fruit.
15:3 Now ye are clean through the word which I have spoken unto you.
15:4 Abide in me, and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me.
15:5 I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing.
15:6 If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned.
15:7 If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you.
15:8 Herein is my Father glorified, that ye bear much fruit; so shall ye be my disciples.

Christ in you.
II Corinthians 5:17 Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.