Harm and reminder undo the effect of a charity because Islam is about spreading the good even with a good word. This money that you spent should not be a reason to harm others.

(… كَالَّذِي يُنفِقُ مَالَهُ رِئَاءَ النَّاسِ وَلَا يُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الْآخِرِ …) (…as does one who spends his wealth [only] to be seen by the people and does not believe in Allah and the Last Day …): This is an example given by Allah (swt) about people who remind and harm when they spend. Everyone knows whether they spend out of hypocrisy or belief in Allah (swt) as well as the Doomsday. The afterlife is mentioned here because this issue is not only related to this world but also the reckoning in the day of judgment. When you od charity out of your money to be known in society, go and get your reward from the people you spend on because you’ve undone your charity. The end of charity is to get the reward from Allah (swt). Then, there is no fear upon you nor about your children; you’re neither sad in the afterlife nor worried about your progeny in this world. The Almighty assures that when you do it in his way. Allah (swt) is so self-sufficient and is needless of having an associate.

“Shaddad Bin Aws (may Allah be pleased with him) narrated that he once was with Prophet Muhammad (saws) and saw that the Prophet was concerned (manifested by his face), so he wondered: ‘O’ Messenger of Allah, your face shows you’re concerned, what is the matter?’ ‘something I fear most about my nation after me.’ ‘what is it?’ Shaddad asked. ‘associating others with Allah and hidden desires.’ ‘O’ Messenger, will your nation associate other with Allah after you?’ Shaddad inquired, and the Prophet replied, ‘O’ Shaddad, they will not worship the sun, the moon, statues or idols but have their deeds to be seen by people[1].”

This is what’s known as a ‘concealed polytheism’ as stated in the Holy Quran:

(… يَا أَيُّهَا الَّذِينَ آمَنُوا لَا تُبْطِلُوا صَدَقَاتِكُم بِالْمَنِّ وَالْأَذَىٰ كَالَّذِي يُنفِقُ مَالَهُ رِئَاءَ النَّاسِ ) (O you who have believed, do not invalidate your charities with reminders or injury as does one who spends his wealth [only] to be seen by the people …): Because the one who spends to be seen by people doesn’t believe in Allah (swt). Nonetheless, even when you deal with the Almighty, your soul may weaken so you remind of or speak about what you have given. That’s why charity in secrecy is better when you fear hypocrisy.

Again, the basic elements of faith are: to believe in Allah (swt) at first and then to believe in the Afterlife, which is very important.

(… فَمَثَلُهُ كَمَثَلِ صَفْوَانٍ عَلَيْهِ تُرَابٌ …) (…His example is like that of a [large] smooth stone upon which is dust …): i.e. big smooth stone.

(… فَأَصَابَهُ وَابِلٌ …) (…hit by a downpour …): Meaning the heavy rain.

(… فَتَرَكَهُ صَلْدًا …) (…That leaves it bare …): Meaning the clear of dust because heavy rainfall clears and makes it smooth. So, it’s as if a charitable person did nothing; Allah (swt) says:

 {وَقَدِمْنَا إِلَىٰ مَا عَمِلُوا مِنْ عَمَلٍ فَجَعَلْنَاهُ هَبَاءً مَّنثُورًا}

(And We will regard what they have done of deeds and make them as dust dispersed.) (Al-Furqaan: 23)

This is what’s meant by the example here: you spend and lose in all ways; you lose money, the one you spend on, and undo the charity.

(… لَّا يَقْدِرُونَ عَلَىٰ شَيْءٍ مِّمَّا كَسَبُوا …) (…They are unable [to keep] anything of what they have earned …): What they earn, use and put in the wrong place is never fruitful.

( وَاللَّهُ لَا يَهْدِي الْقَوْمَ الْكَافِرِينَ … ) (…And Allah does not guide the disbelieving people): Why so? Because they deny and disbelieve in Allah’s verses.

[1] Hadith no. 7940, Vol. 4, P. 366, Al-Mustadrak Ala As-Sahihayn by Al-Hakem.