Saying "In Shaa' Allah" is etiquette of Islam
2-1-2018 | IslamWeb
Question:
Assalaamu alaykum. Is it incorrect to think negatively of the word "In Shaa' Allah" (Allah willing) when something does not happen? I so far have seen that when I say "In Shaa' Allah" to do something, in the end, I do not do it. A very few times, when I say it, it does actually take place. When I do not say it, it sometimes does happen. Please shred light on this issue for me.
Answer:
All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
Saying "In Shaa' Allah" is part of the etiquette of Islam. The Muslim is instructed to say "In Shaa' Allah" whenever he expresses his wish to carry out a future action. Allah, the Exalted, says (what means): {And never say of anything, "Indeed, I will do that tomorrow," Except (when adding), "If Allah wills."} [Quran 18:23-24]
When the person says it in connection to his future actions, these actions may or may not take place; however, it is more hoped that they would take place if he says "in Shaa' Allah."
Shaykh Ibn ‘Uthaymeen 
said, "I advice the Sister who asked the question and all Muslims to add 'In Shaa' Allah' to their oaths to facilitate the fulfillment of their wishes..."
The story of Prophet Sulaymaan (Solomon), may Allah exalt his mention, indicates this meaning. Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet, sallallahu ‘alayhi wa sallam, said, "Sulaymaan said, 'Tonight, I will sleep with (my) seventy wives, each of whom will get a male child who will fight for the cause of Allah.' Upon that, the angel said to him, 'Say, 'In Shaa' Allah,' but Sulaymaan forgot (to say it). He slept with all his wives, but none of them gave birth to a child..." The Messenger of Allah, sallallahu ‘alayhi wa sallam, said, "Had he said 'In Shaa' Allah', he would have been successful and would have attained what he had desired.'" [Al-Bukhaari and Muslim]
This is proof that when someone says "In Shaa' Allah," it is more likely that his wish and need will be fulfilled; however, this is not necessarily the case all the time. If Allah wills it, it shall be fulfilled. It should be noted that deprivation could be a blessing in disguise. Therefore, Allah, the Exalted, says (what means): {But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not.} [Quran 2:216]
What you referred to as "thinking negatively" of this expression is a mistake that resulted from another mistake, which is your assumption that whenever you add it to your statements about future actions and wishes, they shall necessarily materialize.
Allah knows best.