Why does Muslim say "Al-Fatihah" when someone pass away?

Honestly, I don't know either! Based on my research from the internet, there is no hadis (whatever actions or quotes from our prophet Muhammad that has been narrated from trustworthy, specific people) that proves that Rasulullah SAW (prophet Muhammad) has done it. Please tell me if there is. 

In fact, there are few scholar who have the opinion that this practice is unreasonable. To me, from a humble point of view based on my knowledge, as long as it is not against the guide line of Islam, there is nothing wrong with it. Unless it's about a compulsory practice in Islam, that must be based on Al-Quran or Hadis.

However, during school, I remember one of my friend asked our teacher, Ustaz Makhzun (only now I realized how good he is.. :/), 

"Do people that has passed away hear when we recite al-Fatiha?"

I know what she meant was everything we recite. From the prayers, or tahlil (a feast usually done by the family. In Malay culture, it consists of specific reading, usually lead by imam). So here is the answer from my teacher :

It is not about whether they hear it or not. It's about the intention of the reciter, to convey the reward of reciting the Al-Quran with someone who has passed away.

Thus, to lighten the burden of the person, or to show that we love and care for the people who pass away, we can recite any verses of al-Quran, with the intention of sharing the reward.

 Do we have to recite the Al-Fatihah?

The most important thing is, based on a hadis, we have to pray for the best towards the dead. I'm sorry I couldn't find the hadis, please share if you find one! Pray for their well-being, to be forgiven, because that's the only thing we can do.

Recite.

Recite as I have written below, don't just write "al-Fatihah" in the comment section, or just say "al-Fatihah to bla bla bla". It is not a quote, it's not a greeting, it's not something like "salaam" or "my condolence". As a Muslim, you have to recite any of the verses of al-Quran (not only al-Fatihah, you can even read the whole al-Quran!) or prayer sincerely with the intention of lightening the burden of the dead.

For a non-Muslim, thank you so much for noticing about the "al-Fatihah", I'm sorry for any of us giving a wrong conception about al-Fatihah, I hope we are all enlightened a bit from my writing, full of lack.. Nonetheless, you are welcomed to pray for us sincerely, thank you so much. That's what matters more. Still, thank you so much for saying the "al-Fatihah" to remind us all, Muslims, to read it for the dead. Thank you. There's nothing wrong with it. Just sharing some extra knowledge. :)

The meaning of the verses of Al-Fatihah in English

1. Bismillaah ar-Rahman ar-Raheem
In the name of Allah , the Entirely Merciful, the Especially Merciful.
2. Al hamdu lillaahi rabbil ‘alameen
(All) praise is (due) to Allah , Lord of the worlds 
3. Ar-Rahman ar-Raheem Maaliki yaumid Deen
The Entirely Merciful, the Especially Merciful,
4. Maaliki yaumid Deen
Sovereign of the Day of Recompense.
5. Iyyaaka na’abudu wa iyyaaka nasta’een
It is You we worship and You we ask for help.
6. Ihdinas siraatal mustaqeem
Guide us to the straight path
7. Siraatal ladheena an ‘amta’ alaihim, Ghairil maghduubi’ alaihim waladaaleen
The path of those upon whom You have bestowed favor, not of those who have evoked (Your) anger or of those who are astray.
Aameen.

As you can see, Al-Fatihah (or written as Al-Fatiha, سورة الفاتحة) is one of the verses in Al-Quran, the Holy book for Muslim.