Pinchas must have felt left out. His father had been appointed a Kohen, so had his uncles. His grandfather Aharon was the first Kohen and his younger cousins were born into kehuna, so it was just Pinchas who was left out. Why would Hashem do this?

Let's look at Rashi:

   ברית כהנת עולם. שֶׁאַף עַל פִּי שֶׁכְּבָר נִתְּנָה כְהֻנָּה לְזַרְעוֹ שֶׁל אַהֲרֹן, לֹא נִתְּנָה אֶלָּא לְאַהֲרֹן וּלְבָנָיו שֶׁנִּמְשְׁחוּ עִמּוֹ וּלְתוֹלְדוֹתֵיהֶן שֶׁיּוֹלִידוּ אַחַר הַמְשָׁחָתָן, אֲבָל פִּינְחָס שֶׁנּוֹלַד קֹדֶם לָכֵן וְלֹא נִמְשַׁח לֹא בָא לִכְלַל כְּהֻנָּה עַד כָּאן; וְכֵן שָׁנִינוּ בִזְבָחִים (דף ק"א) לֹא נִתְכַּהֵן פִּינְחָס עַד שֶׁהֲרָגוֹ לְזִמְרִי:

The Maharal in Gur Aryeh writes it was a technical matter, Pinchas was a child at the time and we don't anoint a child. In principle he may never have become a Kohen on this technicality and just on the merit of killing Kozbi and Zimri was granted Kehuna.

Rav Gedalia Schorr writes a beautiful explanation on this point, in fact, Hashem had always intended Pinchas to be a Kohen, just Hashem  knew that Pinchas would have the gevura to kill Kozbi and Zimri, and whilst making a great Kiddush Hashem, on a personal level it would make Pinchas possul for Birkas Kohanim.  (Shulchan Aruch O.C. 128:35)  

So Hashem in His kindness delayed Pinchas' appointment as a Kohen so that the killing wouldn't possel him. That's amazing!

With a narrow perspective, the way life works out, can seem not beneficial for us, as if we've been left out.

But Hashem knows what He's doing, you can trust Him.

You may not always see it straight away, but still know, He's watching out for you and it's for your best.

Have a wonderful Shabbos!