Why did the Doctor need the memory worm in the first place? We know that he can erase peoples memories by taking their heads into his hands. Hmm...maybe its pretty stressfull to do it that way?
But he had done the "grabbing someones head and getting into their mind" to someone else before...(don´t remember to whom)..though it didn´t involve memory wiping.
I think he just wanted to touch Reinette, and Who could blame him? As far as the worm, it may have been nothing more than a comedic plot device for Strax, which worked very well.
Who knows how long he's had the memory worm? For all we know, it could've been something he picked up right after losing the Ponds. In a fit of depression, he could've just given it a kiss and forgotten the whole thing. Or, it's just been under the TARDIS console for centuries, stored with all the other stuff filed under "M". Mawdryn, Metebilis crystal, Mel... Mark
Only when Amy's in the console room. Or Rose. Or Peri. Or Romana. Or Romana. Or Leela. Or Sarah-Jane. Or Jo. Or Zoe. Mark