In reply to Andrew Potter:
Ego, ambition, glory, sex, praise, inadequacy, stubbornness, pride, friends, curiosity.
Although to some extent it depends just what you mean by 'climb' and the type of climbing. I doubt motivations have changed much over time, as human nature hasn't.
If you haven't already read it, this is one of the best and most honest pieces that deals with motivation:
http://www.alpinist.com/doc/web17f/wfeature-alp57-threshold-shift
"...the real reason was for the after, for the adulation and acceptance and the slap on the back."