Often when we like or love someone we often might be achieving some sort of benefit from them. Like parents love their children, but expect that they take care of them when they need them. Loving your spouse because he provided for you and takes care of everything for you. Friendships are maintained when people achieve some sort of benefit or help. There is no kind of relation with anyone without expecting anything in return. In every relation we are also giving and receiving something. In the love for the sake of Allah, we are giving un-conditional love to another person, because of our love for Allah. Not expecting anything from anyone, but only because we want to please Him.
There is nothing more pure
When we help and love other for the sake of Allah we fulfil commandments of Allah, that would be pleasing for His creation, in order to please the Creator. Being good to family, parents, friends and neighbours, and fulfilling our obligations. This motivation that keeps us inspired, receiving Allah`s love and never giving up, it`s like a candle that never stops spreading light.
Those that love each other for Allah`s sake will be re-united with each other under His shade on Judgement day. There is no greater achievement. While others are being punished and questioned for their wrongs, the righteous ones are in calmness and joy and rejoicing under His shade. When we want to obey Allah and love His words to the extent that we would want to achieve His pleasure, we would want to find ways to make Him pleased with us. Pleasing His creation for His sake and being patient with them when they are difficult. Realizing that the reward will be given from Him. Even when we do the right thing, some people will hate us for it, other might love us for it. This will change all the time. Even the same person can`t always love us all the time. Therefor as long as we know we are behaving according to the Quran and Sunnah, don`t let people`s negative or positive remarks get us off the right path. Be persistent, and know that the One that will love us, no matter what, all the time, is Allah.
May Allah make us of those righteous people who love each other for the sake of Allah, and are re-united under His shade on Judgement day and in firdous. Aameen summa aameen.