It changed during their time together.
At first, the Doctor seems to have seen Rose as good company -- someone he liked -- but not much more than that. She seems to have regarded him in much the same way & as an opportunity to go to places, to see things & to do things she'd never before thought she could -- a chance to escape from a boring life that seemed a bit of a dead end.
Fairly quickly, they became close friends & friendship (though it's not often spoken of that way) is a form of love.
After they'd been together for a while & had been though some difficult times (including a regeneration), they were obviously in love. That may not have been as obvious to the Doctor as it was to others. Hooking up with a partner is a much more urgent consideration for a short-lived species (like humans) than for a very long-lived species (like Time Lords), so a Time Lord could be expected to be rather "slow on the uptake" in such matters.