1) There is no motivation, asides your goodwill, or possibly wanting to support some useful Bitcoin tools, like the Bitcoin Faucet (transaction fees could eat up a lot of coins there).
2) It's a matter of how their pools and clients are set up. Some want to support the community by allowing no-fee transactions to be included in a block (as it costs them nothing), while others want to encourage people to pay fees by only including transactions with fees.
3) Technically, it might be less likely to be included if it gets really big in terms of data size (upwards to filling a block), but in most cases it won't matter (at least for now). Amount of Bitcoins sent doesn't really matter at the moment, but transactions on really small amount of coins can be discriminated against in the future should spamming them become more prevelent.
All in all, it is encouraged that you pay your dues when sending transactions, as it helps the Bitcoin project grow. If nobody was paying to get their transactions processed, the miners might soon not earn enough to get by (especially with reward halving slowly getting closer), which would result in them stopping their operations and weakening the Bitcoin network. It's not much of a threat yet, but in the future it can become important.