The reason Al likes cats started right after Ed had sealed his soul to the suit of armor. You see, there was an add-on to the reaction.

The person fused with the armor would have a great liking for the first thing they saw when they first woke up. It could even be a thing.

When Al woke up, strangely, a cat was staring at him. So, he really liked the cat. That is the reason Al likes kitties.

He will love cats forever. But, he doesn't mind that at all. Just don't say 'There's more than one way to skin a cat.' He gets really angry, part of the whole 'first thing you see you love' thing.

The end.