i dont think i can answer but will try
Loving for the sake of ALLAH means loving one another for the cause of Allah and only for this not for anything else not for money or becasue he/she is your husband/wife/mother/father but becasue he does what Allah loves so you love him/her for it and much much more i think and also i thik it is a hard thing to get since we are promised to be under the shade on the day no shade will shade them except the throne so for such a great reward i think not anyone can do it
im going to post a few things inshallah they help and if anyone has anything else that is authntic pls share JAZAKUM Allahu khayr
Allah knows best
Those whom Allah loves- Hassan As Somali said in one of his lectures
The person that loves their brother purely for the sake of Allah. In a Hadith reported by Muslim, Abu Hurairah narrated that the Prophet ( sallalahu alaiha wa sallam) said “There was a man who visited his brother who lived in another village, an angel came to him as he was on his was and the angel asked “Where do you plan to go?” he said “I want to visit a brother of mine that resides in that village”, the angel then asked “are you visiting him to repay his favour?”, the man replied “No, the only reason I’m visiting is because I love him for the sake of Allah. The angel then said “I am a messenger that has been sent to you from Allah to inform you that Allah loves you just as you love this person”.
Loving a person for the sake of Allah is not simple so if you find someone who loves you for the sake of Allah hold on to them with all your strength because they are rarer than any gem. In this time people don’t love each other for the sake of Allah. How can you test yourself? One of the Salaf said if when this person treats you well, it dos not increase your love for him and if he hurts you then you don’t hate him more. If you want to see if he is true then upset him, if you see him behaving as you would expect then he is your true friend. Loving one another is something that is rare in our time......
The Prophet (peace and blessings be upon him) said, “A Muslim has six rights over another Muslim.” They asked, “What are they, Messenger of Allah?” He said, “When you meet him, greet him with salam (Muslim's salutation meaning ‘peace’); when he invites you, accept his invitation; when he seeks your advice, advise him; when he sneezes and then praises Allah, invoke Allah’s mercy upon him; when he falls ill, visit him; when he dies, follow his funeral” (Muslim).
The Prophet (peace and blessings be upon him) said, “A Muslim’s supplication for his brother in secret is answered. At his head an angel is appointed, and whenever he supplicates for his brother with something good, the angel appointed to him says, ‘Ameen, and likewise for you’” (Muslim).
(If any of you loved his brother for the sake of Allah, then it is better to tell his brother so that he can feel the same ), As-Silsela As-Sahiha, Al-Albani 1/947.