אָמַר רִבִּי יוֹנָה. יוֹנָה בֶּן אַמִיתַּי מֵעוֹלֵי רְגָלִים הָיָה וְנִכְנַס לְשִׂמְחַת בֵּית שׁוֹאֵבָה וְשָׁרָת עָלָיו רוּחַ הַקּוֹדֶשׁ. לְלַמְּדָךְ שֶׁאֵין רוּחַ הַקּוֹדֶשׁ שׁוֹרָה אֶלָּא עַל לֵב שָׂמֵחַ. מַה טַעֲמָא. וְהָיָה֙ כְּנַגֵּ֣ן הַֽמְנַגֵּ֔ן וַתְּהִ֥י עָלָי֖ו רוּחַ אֱלֹהִים.
Rebbi Jonah said, Jonah ben Amittai belonged to the pilgrims, came to the water-drawing festivity and the holy spirit rested on him, to teach you that the holy spirit only rests on a happy heart. What is the reason? It was when the musical instrument played, God’s spirit was on him.
(טו) הַשִּׂמְחָה שֶׁיִּשְׂמַח אָדָם בַּעֲשִׂיַּת הַמִּצְוָה וּבְאַהֲבַת הָאֵל שֶׁצִּוָּה בָּהֶן, עֲבוֹדָה גְּדוֹלָה הִיא.
וְכָל הַמּוֹנֵעַ עַצְמוֹ מִשִּׂמְחָה זוֹ רָאוּי לְהִפָּרַע מִמֶּנּוּ שֶׁנֶּאֱמַר (דברים כח מז) "תַּחַת אֲשֶׁר לֹא עָבַדְתָּ אֶת ה' אֱלֹהֶיךָ בְּשִׂמְחָה וּבְטוּב לֵבָב".
וְכָל הַמֵּגִיס דַּעְתּוֹ וְחוֹלֵק כָּבוֹד לְעַצְמוֹ וּמִתְכַּבֵּד בְּעֵינָיו בִּמְקוֹמוֹת אֵלּוּ חוֹטֵא וְשׁוֹטֶה. וְעַל זֶה הִזְהִיר שְׁלֹמֹה וְאָמַר (משלי כה ו) "אַל תִּתְהַדַּר לִפְנֵי מֶלֶךְ".
וְכָל הַמַּשְׁפִּיל עַצְמוֹ וּמֵקֵל גּוּפוֹ בִּמְקוֹמוֹת אֵלּוּ הוּא הַגָּדוֹל הַמְכֻבָּד הָעוֹבֵד מֵאַהֲבָה. וְכֵן דָּוִד מֶלֶךְ יִשְׂרָאֵל אָמַר (שמואל ב ו כב) "וּנְקַלֹּתִי עוֹד מִזֹּאת וְהָיִיתִי שָׁפָל בְּעֵינָי". וְאֵין הַגְּדֻלָּה וְהַכָּבוֹד אֶלָּא לִשְׂמֹחַ לִפְנֵי ה' שֶׁנֶּאֱמַר (שמואל ב ו טז) "וְהַמֶּלֶךְ דָּוִד מְפַזֵּז וּמְכַרְכֵּר לִפְנֵי ה'": סָלִיק הִלְכוֹת שׁוֹפָּר סֻכָּה וְלוּלָב
(15) The joy which a person derives from doing good deeds and from loving God, who has commanded us to practise them, is a supreme form of divine worship.
Anyone who refrains from experiencing this joy deserves punishment, as it is written: "Because you have not served the Lord your God with joy and with a glad heart" (Deuteronomy 28:47).
Anyone who is arrogant and insists on self-glory on such occasions is both a sinner and a fool. King Solomon had this in mind when he said: "Do not glorify yourself in the presence of the King" (Proverbs 25:6).
(On the other hand), anyone who humbles himself on such occasions is indeed great and honored, for he serves the Lord out of love. David, King of Israel, expressed this thought when he said: "I will make myself even more contemptible than this, humbling myself in my own eyes" (II Samuel 6:22). True greatness and honor are attained only by rejoicing before the Lord, as it is written: "King David was leaping and dancing before the Lord" (II Samuel 6:16).
A way to gauge the degree of happiness you have in what you're doing is to see how much you care about what others think about what you're doing. When you're energized and happy with what you're doing, other people's opinions don't mean anything; you're too focused on the thing that's making you happy.
This is what the Ramabam is saying to us. One should strive to be happy in his mitzvos to the extent that one forgoes his honor in the process, not caring what others think about your simcha and dedication to Hashem. That is why the Rambam connects the idea of simcha in mitzvos and not being arrogant and self-conscious in times of doing avodas Hashem.
The context behind Yonah going up to be oleh regel is significant. At that time in history, Yerovam ben Nevat had blockaded the path to the Beis HaMikdash with boulders and guards - but Yonah went anyways. Why? Because his Simcha in doing the mitzvos, Aliyah l'rebel among them, overrode the danger and nasty looks he got from his counterparts.
And that is why Yonah got ruach hakodesh, not because he was a "happy spirited person", rather because by going to be oleh regel in such treacherous conditions, it showed just how much simcha he had in his avodas Hashem, and because of that high degree of simcha he merited ruach hakodesh.