Why? Say you created successful online web service. Alone. Than, accidentally (ex. caterpillar behind the next corner) something fatal happen to you. Who will care about running business. Do you think that by ex. PG has appetite to search for new business owner and maintainer? It's not possible to simply jump in the running train if you know nothing about business internals and by Murphy's law one day after your death application stops working due some old, forgotten bugs. Take any Y sponsored site and ask PG what he knows about its source code. This is IMHO the only reason why VC's encourage you to find a co-founder. I'm running my sites alone and still have not a feeling of burn-out :) If you don't have enough skills to implement your idea, employ someone, if you don't have enough time to work on business, employ someone.
Now think about it. If your online business idea is something what cure your customers' pain, you don't need VC's capital to be sucessful. We are living in age of blogs and other wonderful viral marketing tools. Cheap hardware and hosting. Everything what you need is courage, kind community and forget arrogance.
I don't want to say that have a co-founder is bad thing. I just want to say that this must have reason and that reason should not be the VC's opinion.
BTW: find a dependable co-founder is like find a needle in hay stack :)