This question comes from a faulty understanding of the purpose of shmonah esrei. The Rambam (sefer hamitzvos 6, and elsewhere) explains that tefilah (meaning specifically shmonah esrei) is an avodah. The purpose is to serve Hashem, not ourselves. We are having a convesation with Hashem about the ways in which we hope he will interact with the world. Even what may sound like personal bakashos (requests) are tied back into our desire to be able to serve our creator to the best of our abilities and so we ask for forgiveness or understanding not to fulfill a personal need, but as a way for us to be better avdei Hashem.
Therefore we don't daven for children, spouses, parents or any other relationship (why don't we daven for a good Rebbi?!) because our focus needs to be how we can serve God and how creation as a whole yearns to be connected to him.