Someone who is good to you only, may be out of some purpose or interest, rather than sincerely caring about you. They may use your weaknesses, or hurt you without your knowledge. They are good to you, may also be a momentary impulse or novelty, and they may change their attitude or lose interest over time.
Someone who is good in themselves, refers to those who have good qualities and habits, have a positive impact on themselves and others, can express their feelings and needs sincerely, and can respect and understand the feelings and needs of others. They are good to you, because they have such ability and willingness, not because you have anything special.
Finding someone who is good in themselves, can make you feel more secure and trustful, and can also make you get more growth and progress. Because they will not hurt you or use you arbitrarily, nor will they do irresponsible things because of a momentary emotion or impulse. They will support and encourage you with practical actions and words, and will also communicate and solve problems in a reasonable way.
Finding someone who is good in themselves, can also make you understand yourself and love yourself more. Because they will not make you feel that you are valuable only because you are liked by the other party, nor will they make you feel that you have to change yourself for the other party. They will let you see that you have many advantages and potentials in yourself, and will also let you discover that you can do more and better things.