So why does SQL server change execution plan?

Well, some hypotheses:

1. SQL Server has artificial intelligence built in and wants to annoy the developers and DBAs by randomly changing execution plans for the query to make it slow and have developers and DBAs spend extra hours trying to optimize it.
2. SQL Server has artificial intelligence built in and has a bias toward SQL Server performance ocnsulting. Since the rates dropped somewhat it decided it will torture full-time employees until they will hire a consultant with a decent rate.
3. This behaviour was a part of SP4 and was designed to annoy developers and DBAs so they will upgrade to SQL Server 2005 as soon as possible, thus generating much-needed revenue for Microsoft.