Allah, the exalted asks us to rejoice in the great bounty of Islam--Ramadhan being a part of it.

There are many reasons for Ramadhan to be the month of joy. Some of which are mentioned below:

1. For Qur'an (افرح بالقران)

Allah says,

شَهۡرُ رَمَضَانَ ٱلَّذِيٓ أُنزِلَ فِيهِ ٱلۡقُرۡءَانُ هُدٗى لِّلنَّاسِ وَبَيِّنَٰتٖ مِّنَ ٱلۡهُدَىٰ وَٱلۡفُرۡقَانِۚ

The month of Ramadhan [is that] in which was revealed the Qur'an, a guidance for the people and clear proofs of guidance and criterion.
-Surah Al Baqarah, Ayah 185

Shaykhul Islam, Ibn Taimiyyah رحمه الله narrated,
A man once asked one of the Salaf, "How much Qur'an should we read?"
He replied, "Equivalent to the amount of happiness you are looking for."
(Majmoo al Fataawa 7/493)

This shows us that Qur'an has a direct correlation to our happiness; and Ramadhan being the month of Qur'an is inevitably the month of joy!

2. For Fasting (افرح بالصيام)

Allah says,

يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُواْ كُتِبَ عَلَيۡكُمُ ٱلصِّيَامُ كَمَا كُتِبَ عَلَى ٱلَّذِينَ مِن قَبۡلِكُمۡ لَعَلَّكُمۡ تَتَّقُونَ

O you who have believed, decreed upon you is fasting as it was decreed upon those before you that you may adopt taqwa (God-conscious).
-Surah Al Baqarah, Ayah 183

So, when we enter Ramadhan, we enter the school of taqwa. Not everyone graduates from this school; hence, if we do, it is worthy of being happy!

3. For Repentance (افرح بالتوبة)

Abdur Rahman ibn Awf reported: The Messenger of Allah SA said, “Verily, Ramadan is a month in which Allah Almighty has obligated its fasting. I have instituted for Muslims the practice of prayer at night. Thus, whoever fasts it with faith and expecting reward will be rid of sins like the day he was born from his mother.”
(Musnad Ahmad 1691, authenticated by Ahmad Shakir)

Imagine having your sins forgiven as if you were born on that very day. Isn't it enough of a reason for us to be joyful?

4. For Mercy (افرح بالرحمة)

Abu Huraira reported: The Messenger of Allah SA said when the month of Ramadan arrived, “The month of Ramadan has come, a blessed month in which Allah Almighty has obligated you to fast. In it the gates of the heavens are opened, and in it the gates of Hellfire are closed, and in it the devils are chained, and in it is a night that is better than a thousand months. Thus, whoever is deprived of its good is truly deprived.”
(Musnad Aḥmad 7108, authenticated by Ahmad Shakir)

Ramadhan is a month filled with mercy for our salvation. The mercy he shows in this month is unequivocal to any. Therefore, this month deserves to be welcomed with nothing but happiness.

5. For Intercession (افرح بالشقاعة)

Abdullah ibn Amr reported: The Messenger of Allah SA said, “Fasting and the Quran will intercede for the servant on the Day of Resurrection. Fasting will say: O Lord, I prevented him from food and drink during the day, so let me intercede for him. The Quran will say: O Lord, I prevented him from sleeping during the night, so let me intercede for him. Thus, they will both intercede for him.”
(Musnad Ahmad 6589, authenticated by Ahmad Shakir)

Subhaan Allah, lawyers persisting and interceding for us on a day when even our own mothers will desert us. Shouldn't we be grateful and happy for a month that gives us this opportunity?

6. For Laylatul Qadr (افرح بالليلة القدر)

Allah says,

إِنَّآ أَنزَلۡنَٰهُ فِي لَيۡلَةِ ٱلۡقَدۡرِ. وَمَآ أَدۡرَىٰكَ مَا لَيۡلَةُ ٱلۡقَدۡرِ. لَيۡلَةُ ٱلۡقَدۡرِ خَيۡرٞ مِّنۡ أَلۡفِ شَهۡرٖ

Indeed, We sent the Qur'an down during the Night of Decree. And what can make you know what is the Night of Decree? The Night of Decree is better than a thousand months.
-Surah Al Qadr, Ayah 1-3

Abu Huraira reported: The Messenger of Allah SA said, “Whoever stands in prayer during the Night of Decree due to faith and seeking reward, his previous sins will be forgiven.”
(Saheeh Bukhari 1802, Saheeh Muslim 760)

Imagine worshipping Allah for one night and receiving reward for worshipping Allah for a 1000 months. Better still, having your sins forgiven completely. What better than to welcome the month which brings this to us in joy!

7. For Rewards (افرح بالأجور)

Abu Huraira reported: The Messenger of Allah SA said, “Every deed of the son of Adam is multiplied, a single deed as ten times the like of it up to seven hundred times. Allah Almighty said, ‘Except for fasting, as it is for Me and I will reward it. He leaves his desires and his food for My sake.’”
(Saheeh Bukhari 1795, Saheeh Muslim 1151)

If Allah, the most Generous says that He is going to exclusively reward for it, we can just conjecture how magnificent its going to be.

There are many more virtues and blessings of Ramadhan which cannot possibly be enumerated in this small space. However, getting a glimpse of the majesty of this month should be enough to convince us that this month of happiness indeed needs to be welcomed with nothing but pure joy!