After describing the centrality of a Mikdash as the one place where korbanos should be offered after the conquest of Eretz Yisrael (12:6):
וַהֲבֵאתֶם שָׁמָּה, עֹלֹתֵיכֶם וְזִבְחֵיכֶם, וְאֵת מַעְשְׂרֹתֵיכֶם, וְאֵת תְּרוּמַת יֶדְכֶם; וְנִדְרֵיכֶם, וְנִדְבֹתֵיכֶם, וּבְכֹרֹת בְּקַרְכֶם, וְצֹאנְכֶם.
The Torah continues and relates that not only obligatory korbanos, but also ma'aser, bechoros, and other pledges should be brought to the Mikdash (12:17):
לֹא-תוּכַל לֶאֱכֹל בִּשְׁעָרֶיךָ, מַעְשַׂר דְּגָנְךָ וְתִירֹשְׁךָ וְיִצְהָרֶךָ, וּבְכֹרֹת בְּקָרְךָ, וְצֹאנֶךָ; וְכָל-נְדָרֶיךָ אֲשֶׁר תִּדֹּר, וְנִדְבֹתֶיךָ וּתְרוּמַת יָדֶךָ.
The literal translation of this pasuk is "you cannot eat" ma'aser, bechoros, etc. wherever you please. However, as we all learned in English grammar, "cannot" implies something is impossible; "may not" is the proper term to use when an action is illegal or not permissible. Rashi apparently was sensitive to the difference and therefore cites R' Yehoshua ben Korcha: "You can eat, but are not permitted to." So much for the correct meaning of the pasuk, but it does not address the underlying question. Why does the Torah use the expression, "lo tuchal l'echol" -- "you cannot eat..." with the extra word "tuchal" connoting "cannot", instead of simply saying "lo yochal...", etc. -- "you may not eat..." as it does for all other eating prohibitions?
I'll leave it up to you to take a peek at the commentaries on Rashi who explain the pshat; I want to share a thought of the Tiferes Shlomo. A person who comes to Yerushalayim to offer korbanos is not just there to have a steak. The experience of being in the Beis haMikdash, partaking of food which is from "shulchan gavoha", is a spiritual one which has no comparison. What happens after you walk away from such an experience? If the experience is authentic, real, something which penetrates to your soul, then there is no way you can walk away saying, "OK, I had my week vacation in the Mikdash, now back to the regular routine." It's just not possible -- "lo tuchal l'echol," you just can't go home and stop at the Kosher Delight for a hamburger and fries, you just can't even eat your ma'aser the same way, because you now know what eating really means, what it means to taste food with your neshoma and not with your guf, and it's that true eating which you will forever yearn to re-experience again.
A practical suggestion: We unfortunately do not yet have the opportunity to visit the Mikdash and eat korbanos, but we do have the opportunity to visit our local mikdash me'at, our shuls, and engage in tefilah, the substitute for korbanos. Let's be realistic: it is hard during the week, when there is so much going on, to daven with kavanah, to stay focused on every word, not rushing to blast out of shul. Even if you personally daven that way, odds are if you daven outside a yeshiva setting, the rest of the minyan is rushing out the door. I have even heard of a "matzah minyan" -- 18 minutes, start to finish. How can we improve the quality of our davening? I think the answer lies in how we daven on Shabbos, and if possible, on Sunday. On those days there is no rush, there is not a train or job waiting, there is nothing to stop anyone from making tefilah into the experience it should be. If davening on Shabbos and Sunday is a davening that takes you to spiritual heights, then during the week you will yearn to re-experience at least a taste of that same type of davening. You just can't experience tefilah like it was meant to be and then go back to the normal routine the rest of the week. Of course, there will always be the bus, the train, the job, but I think there will be a renewed appreciation and desire for tefilah that will make a difference.
Let me describe what I mean by a davening that is a spiritual experience because I am cynical by nature and was just amazed to find such a thing in real life. I was recently at a minyan where over 80% of the attendees were already seated and present before brachos. Once the shat"z started, if someone needed something from a neighbor, e.g. to squeeze by their seat, to get a sefer, etc. they simply gestured, motioned, etc., but there was no talking. There were no mishebeirachs between aliyos, but there was no talking bein gavra l'gavra of kri'as haTorah. There was not a sound during shmoneh esrei, and no one started a conversation if they finished early and were waiting for the shat"z to start chazaras hashat"z. It was simply quiet, except for the shared voice of tefilah. The Rabbi did not shush people, the gabay did not klap the bimah every two minutes for quiet, kids who were there sat reading or went out if they needed something. This is the yotzei min haklal which is melameid that my cynical attitude is wrong and the same achievement is possible even for the rest of the klal. Let's be real: there are people who simply do not see any purpose for coming to shul if they cannot speak to their neighbor, have kiddush whenever they feel like, drag their kids along for free babysitting for a few hours and lots of candy, walk out on the Rabbis derasha to do whatever, and generally turn a religious experience into a social experience (see, my cynical side is taking over again). But it is possible for things to not be this way, and baruch Hashem that some people do get it, because the difference is not just a difference in Shabbos davening, but is a difference that echoes through the week as well.
The month of Av is the month in which we lost the Beis haMikdash. The key to getting it back is to show that we can't live without it, without meaningful avodah as it should be.