These were my feelings too when I saw the regeneration in Series 1. I still feel this way whenever I watch it.
He was gone.
Her Doctor was gone and in his place was this skinny, messy-haired man. He was insisting that he was the Doctor but how could he be?
Her Doctor had exploded into bright, golden light, leaving this grinning interloper in his place. Yes he was wearing the same clothing that the Doctor had been but the rest of him was completely different.
Her Doctor never babbled on about things that had no relevance. And now this intruder was taking her hand, whispering things to her that only her Doctor would know. He couldn't be her Doctor. Could he?