why couldn't G-d answer the angels himself?

The Torah always puts the mitzvah of kibud av next to the mitzvah of Shabbos: you have it in the aseres hadibros in our parsha, you have it in the second dibros, you have it in Parshas Kedoshim - "Ish imo v'aviv tira'u v'es shabsosei tishmoru."  What's the connection?  Chidah in Nachal Kedumim explains that on Shabbos one has time to learn in greater depth.  When one is mechadesh in Torah on Shabbos it causes one's parents who are in the next world to be rewarded with a crown to wear, a special spiritual boost.  Shabbos thus provides the opportunity to fulfill kibud av even when one's parents are no longer here.  In keeping with that idea, please learn the divrei Torah for this Shabbos l'zecher nishmas my father, whose yahrzeit will be this upcoming week.  

The gemara (Nidah 70) writes that the people of Alexandria in Egypt sent 3 questions of derech eretz to R' Chanina ben Gamliel: how to become smart, how to become rich, and how to have good kids.  (We would all love the answers to those too, right?)    The first question was how to attain chochma, to which R' Chanina answered that the key is to spend a lot of time in yeshivah learning.  They replied that this can't be it, as a lot of people tried that and have not been successful.  R' Chanina responded that there is another ingredient needed as well: you have to daven.  The gemara concludes that you need both factors, both yeshiva and davning, and one without the other won't work.

The second question was how to get rich, and R' Chanina answered that the key is to spend a lot of time doing business (GR"A takes that out!) and to do so honestly.  Again, the people of Alexandria argued that a lot of people tried that and failed, and again, R' Chanina told them there is another ingredient necessary: tefilah.  The gemara here too concludes that you need both ingredients together.

The third question was how to have "banim zecharim," and R' Chanina told them that the key was to marry a good wife and to act with tzeniuyus.  You can guess the rest of the shakla v'terya at this point : )

I have a one simple question.  If the answer to each question was that you need X + tefilah, why didn't R' Chanina say that?  Why did he give the people of Alexandria only half the recipe at first, and only when they pressed the point and complained that the recipe did not work did he reveal to them that they needed to daven as well?  

The Maharal in his Derush al HaTorah says a fantastic yesod (in Tif Yisrael he says a different answer) that is worth knowing even if you don't think it explains R' Chanina's answers.  The gemara famously tells us that when Moshe went up to get the Torah, the angels complained and asked what this human being is doing up in Heaven, in their place.  Mankind does not deserve Torah and should stay back down on earth.   Moshe Rabeinu was afraid of what the angels might do to him, but Hashem responded that Moshe should just hold on to His throne and answer them, which he did.  Maharal asks: why did Hashem put Moshe on the spot and tell him to answer the angels?  Moshe was there on Har Sinai only because Hashem and told him to come up the mountain to get the Torah.  This was Hashem's plan -- why didn't Hashem answer the angels himself?!

Let me put the Maharal's answer in contemporary terms.  A guy or girl might go out on a date because their parents think the boy/girl would be a good match, their rebbe or morah thinks the boy/girl is a good match, their friends think the boy/girl is a good match.  But when that boy or girl is standing under the chuppah, if you ask him/her what they are doing there, oy va'voy if all they can say is, "My mother/father/rebbe/morah/friend thinks this is a good match."  If they can't say, "I can't think of how I can live without Bas Ploni/Ben Ploni; without him/her my life is incomplete," then l'fi anyiyus da'ati there is something very wrong.  When you are under the chuppah, no one else's sevara works to explain what you are doing there. 

"Yom chasunaso," explains the Mishna in Ta'anis, is the day of mattan Torah.  When the angels came to ask Moshe what he was doing there, no one else could answer that question.  G-d himself can't answer that question for him!  If you want to be mekabeil the Torah, the answer has to come from you.  Torah is the key to achieving perfection, but unless YOU first feel imperfect, lacking, incomplete, without it, you don't really deserve it. 

There are a lot of kids who sit in yeshiva listening to their rebbe drone on in shiur, totally bored out of their minds, feeling like it's a jail sentence.  If you ask them, "What are you doing here?" the answer these kids will give probably will have to do with where their parents want them to go, what is expected, lack of other choices, etc.  They don't feel like they would be missing anything by not being there.  You can't be mekabeil Torah unless you feel like you are missing something without it. 

Getting back to R' Chanina, I think the gemara deliberately left tefilah off the list of ingredients.  R' Chanina wanted the people coming back feeling, "We've tried everything you said and it hasn't worked for us."  He wanted them to acknowledge that their best efforts were not just goof enough.  Until they felt that there was something more needed beyond what they could muster on their own, until they felt their efforts were incomplete without it, they weren't ready for tefilah. 

So when you sit in shul and listen to Parshas Yisro, the question to ask yourself is, "What am I doing here?"  Hopefully the answer is not about your neighbors, your husband/wife's expectations, your kids, or your social position.  Hopefully it's about how you find fulfillment and where you find fulfillment.