Why does my trait need a lifetime parameter?

Being a Rust newbie, I probably somewhat naively started with this:

...

pub trait Decode<T> {
    fn decode_from<R: io::Read + ?Sized>(&mut self, stream: &mut R) -> T;
}

pub struct MQTTFrame<'a> {
    pub payload: &'a Vec<u8>,
}

pub struct MQTTFrameDecoder<'a> {
    pub payload: &'a mut Vec<u8>,
}

impl<'a> Decode<MQTTFrame<'a>> for MQTTFrameDecoder<'a> {
    fn decode_from<R: io::Read + ?Sized>(&mut self, stream: &mut R) ->    MQTTFrame<'a> {
        stream.read(&mut self.payload);
        MQTTFrame{ payload: self.payload }
    }
}

Which, when trying to compile, was greeted with:

src/testbed/mod.rs:31:24: 31:36 error: cannot infer an appropriate   lifetime for automatic coercion due to conflicting requirements [E0495]
src/testbed/mod.rs:31         MQTTFrame{ payload: self.payload }
                                                  ^~~~~~~~~~~~
src/testbed/mod.rs:29:5: 32:6 help: consider using an explicit lifetime  parameter as shown: fn decode_from<R: io::Read + ?Sized>(&'a mut self,   stream: &mut R)
 -> MQTTFrame<'a>
src/testbed/mod.rs:29     fn decode_from<R: io::Read + ?Sized>(&mut self, stream: &mut R) -> MQTTFrame<'a> {
src/testbed/mod.rs:30         stream.read(&mut self.payload);
src/testbed/mod.rs:31         MQTTFrame{ payload: self.payload }
src/testbed/mod.rs:32     }

Somewhere on StackOverflow - sorry, I forgot where - someone in a similar case suggested to add a lifetime parameter like so (omitting unchanged code):

pub trait Decode<'a, T> {
    fn decode_from<R: io::Read + ?Sized>(&'a mut self, stream: &mut R) -> T;
}

impl<'a> Decode<'a, MQTTFrame<'a>> for MQTTFrameDecoder<'a> {
    fn decode_from<R: io::Read + ?Sized>(&'a mut self, stream: &mut R) -> MQTTFrame<'a> {
        stream.read(&mut self.payload);
        MQTTFrame{ payload: self.payload }
    }
}

And lo and behold! It compiles. Now if I could only understand why it compiles. Could someone explain

1. why the original code did not compile?
2. why the modified code compiles?