Shulchan Aruch Orach Chayim 61:4:
We are accustomed to reading the first verse [of "Sh'ma"] in a loud voice in order to arouse the direction [of our thoughts].
SA OC 101:2:
But he should not make his voice heard [when saying sh'mone esre]. But if he can't direct [his thoughts] silently, it's permitted. However, that's only when he's alone: while with the community it's forbidden, as he'll come to disturb the community.
(For practical halacha, CYLOR; certainly don't count on my quoting or translating these correctly. Note also that I didn't quote the commentaries on the SA.)
Why is it that "Sh'ma" we say aloud to effect kavana (direction of thoughts) while sh'mone esre we say quietly so as not to disturb others? Why doesn't the same rule apply to both? What determines that not affecting others is more important for sh'mone esre but effecting one's own kavana is more important for "Sh'ma"?