You had been training this Mudkip for the longest time, it was your starter and you and Mudkip had a lovely bond..that was until he met your rival. Your Mudkip was always so happy to see your rival that it seemed like it would really be happier WITH the rival. Of course you and Mudkip had that forever bond..it just- maybe you were thinking this through too much? Mudkip would always favor you..right?