Why does the XP nodelet say that I have some number of dog votes left?

If the majority of the last several votes cast by you were down-votes (and if you still have votes left to cast today), then your XP Nodelet will contain a link to this FAQlet. This gives you a visual cue that casting more down-votes will incur an increasing chance (up to a maximum of 1/3) of costing you 1 XP per down-vote you cast.

If you find the prospect of losing a few XP distasteful, then you will need to find at least a few more-deserving nodes that you can up-vote to get your "voting average" back above zero for a while. Also, you may want to consider being less quick to down-vote nodes (considering how distasteful you find losing XP to be and since your down-votes cause others to lose XP). Down-voting nodes for relatively minor reasons can often be disruptive to our community.

Also see Voting/Experience System. For explanatory background, see History now influences voting.