Like a lot of PPs, for me it depends on the relationship I have with the person as well as the amount.
In general, if it is less than £1 then I will make an effort to pay it if I am the borrower, but I won't much care if I'm not paid back, but I will notice if there is a pattern of someone never offering, and it would definitely make me much less likely to lend them anything larger.
(and if I offer to pay back, or to give someone money if they've paid for a coffee from the machine at that kind of price, and they say no, I will accept that at face value and move on) If they do pay back, it's a nice surprise and yes, it does make me feel that person is more trustworthy.
The only exception would be if the borrower is someone who makes a habit of 'forgetting' to return money.
For larger sums, I would expect to be paid back / given my change unless its a situation with close family / friends where I feel it will all work out over all, but in any situation where someone hands money to me to buy something on their behalf I would always offer them their change back
It does irritate me if someone says they will pay you back and then doesn't - I recall not long after I left university, a woman I'd shared a house with came round to see me. She stayed longer than she'd intended and missed the bus she'd planned to catch, so she asked me if I would lend her £5 for a taxi. At the time, this was not 'I can't pay my bills' money for me but it was a noticable amount in my budget. She told me she would post it back to me by the end of the week.
I bumped into her about a month later, and she immediately said "Oh, I haven't forgotten that £5, I'll post it to you"
6 months later, she sent me a Christmas card and wrote in the card "I haven't forgotten that £5 I owe you" (but didn't enclose it in the card)
By that time I'd written it off and just made a mental note never to lend her anything again, but the card really annoyed me. If you can go to the trouble of writing, and posting a card, and setting out that you owe me money, you can put the money in the darn card. If you don't, I'm going to assume that you are making a conscious choice not to pay your debt, whereas otherwise I'd assume you'd forgotten.
(Interestingly, I subsequently found out that the same woman was dishonest in other ways, too. Nothing criminal, just in how she treated other people)