Here's the thing: If you're doing favors for others, complimenting them, and buying things or making small gestures for them WITH THE INTENT & EXPECTATION OF EARNING THEIR AFFECTION, you are NOT actually a nice guy. If you were truly a nice person, 1. you wouldn't be doing these things with an expectation of something in return and 2. you wouldn't want someone you care about to feel obligated to you.

If you're not making your romantic interest clear and upfront, then (at best) they probably just see you as a very generous or kind friend. It's not their fault for not knowing your intentions if you never made them clear. If anything, it's cowardly not to because you're hiding behind the shield of plausible deniability: if they end up catching feelings, then you admit that you're interested in them and you get together. If she doesn't feel the same way, then you say you're just trying to be a good friend. Either way, it's cowardly and manipulative and you only have yourself to blame if you get friendzoned.

Frequent "nice" gestures towards an individual (and no one else) can actually be really confusing and can send a mixed message. If the other person isn't romantically interested in you, then it puts them in an awkward position (trying to figure out if you're actually "nice" or if you have some ulterior motive). Friendzoning, especially in cases like this, can actually be a way to subtly communicate the lack of romantic desire, but appreciation of your character and friendship.

If you like someone, make it clear. Take the guesswork out and make everyone's lives easier. You'll save yourself money, time, and effort and you won't be that creepy friend or coworker that's harboring a crush but is caught in the friend zone (I really wish I could tell this to my younger self).

ALSO, do nice things to be a nice person, not to get something back or in exchange. True kindness is attractive, but manipulation is not.