I can imagine 10 not being a very big fan of 9. While 9 still had a sense of humor, he was definitely much more grounded and serious about his tasks than 10. 10 would have probably found 9 boring (David Tennant, Tenth Doctor, why has nobody made that pun yet?)
Actualy, I think 2 had a harder time comming to terms with his "death" than 4, infact I didn't even notice his fear of regenerating.