To regression test, or not to regression test

Building up a proper testing suite can involve a good amount of work, which I'd prefer to avoid because it's boring and I'm lazy.

On the other hand, if I'm not careful, taking shortcuts that save effort in the short run could lead to a massive productivity hit further down the road.

For example, let's say instead of building up a rigorous test suite I test my code manually, and give a lot of careful thought to its correctness.

Right now I think I'm OK. But how long will it stay that way?