Have you noticed when you meet a particular brother or sister for the first time, you get along with them instantly. You agree on most things, you feel comfortable in their presence and most importantly you talk to them as if you have known them for years. You even express yourself in a way that would take you months or even years with other people. You don't hold back what you want to say and can express yourself freely in their presence.
After meeting them a few times you have a firm attachment to them and proudly proclaim them to be your brother or sister. The Prophet (sallallahu alayhi wasallam) described why this happens.
A'isha (radiallahu anha) narrated that the Prophet (sallallahu alayhi wasallam) said: “Souls are troops collected together and those who got along with one another (in the realm where souls existed before entering physical bodies in this world) will have an affinity with one another and get along (in this world)."
(Bukhari, 3158; Muslim, 2638)
So the brothers and sisters that you love and cherish in this world are with you for a reason. You were joined together before you were born and you exist in harmony together as brothers in this world as a result of that attachment.
Al-Khattabi states that people are attracted to those who are like them and people’s souls recognise one another according to their nature. If they are similar they will get along and if they are different they will not get along.
Finally this sums things up. Ibn Hibbaan narrated: "Ibn Abbas saw a man and said, ‘Indeed he loves me.’ His companions asked, ‘And how do you know?’ He said, ‘Because I love him and souls are like recruited soldiers, those that recognise one another unite in harmony and those that do not recognise one another are at an aversion.’
(Rawdah Al-Uqalaa, 1/108).
May Allah bless our brothers and sisters and may we all meet in the gardens of Jannah under the Shade of Allah.