When is it okay to ask for a favor/when is it better to be polite

I'm not sure when it's okay to ask for a favor and when it's better to be polite and not ask. I've noticed my friend doing this:

1. A grape tomato drops off of my pizza slice. Friend: Can I have that?
What I would do: *Not ask*

2. We are walking together. I will never take the lead or have the guts to ask: "Can we stop at the book store?" "Can I get a coffee?" or even simply just walk over to a bench and sit down.

This borders on self-esteem issues. Is most SA the result of low self-esteem? For me, my low self-esteem may actually be biological. I'm naturally overly cautious and sensitive to danger and I may construe my anxiety as low self-esteem when in reality, it's simply my brain being oversensitive and making me feel bad and insecure because I'm wired to be overly cautious.

"Asking" for favors is always fine. Only when you "Demand" them, you are being inappropriate or rude. Example - Asking your friend for the tomato gives your friend the option to say "yes" or "no". Demanding, which would be saying "give me the tomato" is disrespectful. You can "ask" for favors just don't use them to take advantage of a person, obviously.. i'm sure you know that much.