About a Keith Richards’ quote on songwriting…
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Keith Richards, on songwriting:
“What is it that makes you want to write songs? In a way you want to stretch yourself into other people’s hearts. You want to plant yourself there, or at least get a resonance, where other people become a bigger instrument than the one you’re playing. It becomes almost an obsession to touch other people. To write a song that is remembered and taken to heart is a connection, a touching of bases. A thread that runs through all of us. A stab to the heart. Sometimes I think songwriting is about tightening the heartstrings as much as possible without bringing on a heart attack.”

From Wikipedia:
Jagger–Richards (spelled Jagger–Richard from 1963 to 1978) is the songwriting partnership between English musicians Mick Jagger and Keith Richards, of the Rolling Stones. They are one of the most successful songwriting partnerships in history. In addition to Jagger and Richards’ songwriting partnership, they have also produced or co-produced numerous Rolling Stones albums under the pseudonym The Glimmer Twins.
Similar to contemporary songwriting partnership of John Lennon and Paul McCartney, both Jagger and Richards write lyrics and music.
Jagger and Richards have different recollections about their first songwriting endeavours but both credit manager Andrew Loog Oldham as the catalyst for their collaboration. 
Richards agrees that it was Oldham who pressed the pair to write songs after the duo had first emphasized other people’s material; Oldham noted that there weren’t that many obscure great songs out there for the band to cover.
So what Andrew Oldham did was lock us up in the kitchen for a night and say, “Don’t come out without a song.” We sat around and came up with “As Tears Go By”. It was unlike most Rolling Stones material, but that’s what happens when you write songs, you immediately fly to some other realm. The weird thing is that Andrew found Marianne Faithfull at the same time, bunged it to her and it was a fuckin’ hit for her – we were songwriters already! But it took the rest of that year to dare to write anything for the Stones.