I have an 11 month old Cocker Spaniel who is generally not agressive but over the last 2 or 3 weeks developed a bit of an attitude when we are out walking.  It has only happened 4 times but it is worrying and I don't know what the cause could be.  On the first occasion we were walking in the fields and we saw two other dogs which he knows he started after a couple of minutes snarling at one of them.  I didn't at the time take much notice but it happened again when I went to see from friend and go for a walk with her dog.  Alfie my cocker started snarling and going for this golden retrievers neck I put it down to being on the lead because as soon as the dogs were let off he was fine and played with my friends dog quite happily.  The next time he did this was when I went to my friends house for the evening I took Alfie and when we arrived he immediately started snarling and trying to bite his neck, it got some bad that I took him home.  During this time at no point did my friends dog react.  The last time Alfie did this was while walking the other day and we bumped into Bailey another golden retriever, Alfie has come into contact with Bailey on several occasions and has been fine and to start with on this occasion he was ok, then suddenly the attitude started and he became extremely agressive and trying to bite his neck.  I thought it might be down to hormones and have booked him in to be castrated but now am having second thoughts.  Can anyone enlighten me as to this problem.
Thanks

The best thing to start with would be to rule out any medical reason for his behaviour by having a vet check him out - he could be uncomfortable and reluctant to be 'bounced' by another dog.  There can be so many reasons for dog/dog agression it would be inappropriate to advise over the 'net, and your best bet would be to ask your vet to refer you to a behaviourist, preferably a member of the APBC or UKRCB.

My dogs do this ALL the time. Whenever they walk by a friends mastiff they snarle , jump and try to nibble the neck , i've never thought anything about it besides giving a NO and moving on. I think it's all good unless there's growling going on.