Let us take a look at the life of a Muslim whose way of life has equipped him with a completely unique perspective which keeps him smiling at all times.
When cash is a little low, the believer is able to remain happy because he reads in the Qur’ān,
“And there is not a moving creature on earth except that it is upon Allāh to provide for it.”
When somebody tries to degrade him, he is able to remain happy because he reads in the Qur’ān, “and dignity is for Allāh, His messenger and to the believers.”
When he finds himself alone and without companionship, he is able to remain happy because he reads in the Qur’ān, “Do not be sad, Allāh is with us”, and he reads, “Your Lord has not forsaken you nor does He hate you.”
When he feels under threat and vulnerable, he is able to remain happy because he reads in the Qur’ān, “Is Allāh not sufficient for His servants?”
When he feels no one understands him and no one is willing to listen to his problems, he is able to remain happy because he reads in the Qur’ān, “I only complain of my grief and sorrow to Allāh”.
When any difficulty arrives at the doorstep of a believer, he is able to remain happy as he reads in the Qur’ān, “with difficulty is ease.”
This is the peace of mind which Islām gives its people, just as the Prophet (sall Allāhu ʿalayhi wa sallam) said,
“How strange is the affair of the believer, as all of his affairs are good! If he is struck with goodness, he shows gratitude and this is good for him, and if he is struck with difficulty, he shows patience and this is good for him” 
Thus, if someone asks you to summarise the happy life of a Muslim, respond by saying: “Gratitude during times of ease, patience during times of hardship, and the gardens of paradise in the end.” This is the beautiful life of a Muslim we wish to share with the entire world.
The life of a practicing believer is one of unexplainable bliss because he is pleased with every one of Allāh’s decrees. Happiness cannot be attained in the least without firm belief in Qadar/predestination, it is a cornerstone to happiness. The Prophet Muḥammad (sall Allāhu ʿalayhi wa sallam) also taught us what to say in various situations.
When the Muslim wakes up in the morning, he has been taught to say:
“All praise and thanks are due to Allāhwho gave us life after our dying and to Him will be the resurrection.”
When the Muslim goes to bed at night, he has been taught to say:
“All praise and thanks are due to Allāhwho has given us food and drink, and given us enough and refuge, for how many people have neither.”
When a Muslim puts on new clothes, he has been taught to say:
“All praise and thanks are due to Allāh, You clothed me with this.”
When the Muslims exits the bathroom, he has been taught to say:
“All praise and thanks are due to Allāhwho removed harm from me and gave me wellbeing.”
When the Muslim sees a person with a disability, he has been taught to say:
“All praise and thanks are due to Allāh who has spared me from what He has trialled you with and has favoured me over you and over many whom He has created a great favouring.”
When the Muslim achieves a matter he wished to accomplish, what has he been taught to say?
“All praise and thanks are due to Allah, whom, by His favour, all good things are achieved.” 
And when a Muslim fails to achieve a matter that he wanted to accomplish, he has been taught to say:
“All praise and thanks are due to Allāh in all circumstances.”
And so the Muslim finds himself shifting from one form of gratitude to another, from one form of contentment to another, regardless of the outcome or situation at hand.
In the absence of this mentality pains become unbearable, calamities become intolerable and life becomes like the one who consumes salty water; theirs is a thirst that can never be satisfied. Such a person finds himself desperately alternating from one avenue of sin to another, from one bed to another, from one drink to another, from one relationship to another, from one online video of obscenity to another, in desperate search for peace and lasting happiness, but his distance from Allāh only causes his heart to further tighten and his life to further darken. He craves to fill the gaps within his heart, but this is not how they are to be filled.
Imām Ibnul Qayyim said,
“In the heart of every human being, there is a sense of scattering which can only be gathered by turning to Allāh, and in the heart there is a sense of loneliness which can only be filled by being close to Allāh, and in the heart there is a fear and anxiety, which can only be removed by escaping to Allāh, and in the heart there is a sense of regret, which can only be removed by being content with Allāh.”
By looking at these examples, we discover something profound. We realise that true peace of mind and the fullest meaning of happiness is in one’s closeness to Allāh and distance from His prohibitions. It is that simple, as Allāh said,
“Whoever does good whether male or female and he is a believer, We will most certainly make him live a happy life.”
And Allāh said,
“… then whoever follows My Guidance he shall neither go astray, nor shall be distressed. But whosoever turns away from My Reminder, then for him is a life of hardship, and We shall raise him up blind on the Day of Resurrection.”
We can read all the mainstream theories, take every medicine on the shelf and try knocking on every door in search for this gem called happiness, but it will not be found anywhere except where the Creator of happiness has taught us to find it. That is because,
“It is He who grants laughter and tears.”
We are happy; happy that we know Allāh, happy with Him as our Lord. We are happy that the Qur’ān is our guidance in life and that we have not been left at the mercy of the ideas of man. We are happy that we will be the first to be held accountable on day of Judgement and the first of all nations to enter Paradise.
 Al-Qur’an, Surah 11, Ayah 6
 Al-Qur’an, Surah 63, Ayah 8
 Al-Qur’an, Surah 7, Ayah 40
 Al-Qur’an, Surah 93, Ayah 3
 Al-Qur’an, Surah 39, Ayah 36
 Al-Qur’an, Surah 12, Ayah 86
 Al-Qur’an, Surah 94, Ayah 5
 Muslim, on the authority of Suhaib
 Bukhari, on the authority of Hudhaifa
 Muslim, on the authority of Anas
 AtTirmidhi, on the authority of Abu Sa’eed
 Musannaf Ibnu Abi Shayba, on the authority of Abu Dharr (Mawqoof)
 Musannaf Ibnu Abi Shayba, on the authority of Ibnu ‘Umar (Mawqoof)
 Al-Adkhaar of Imam AnNawawi, on the authority of ‘Aisha
 Al-Qur’an, Surah 16, Ayah 97
 Al-Qur’an, Surah 20, Aayaat 123-124
 Al-Qur’an, Surah 52, Ayah 43