Pokemon Nature, for simplicity, has switche roles. Rather than directly affecting your pokemon's growth, Nature is now focused toward friendship. Certain Natures are recommended over others. The nature does still dictate which color treats they like (Treats: Pokeblocks, Poffins, Pokesnacks). For those of you that don't know the natures, they are...
Adamant- Likes Red Hates Blue
Bashful- Likes no specific color
Bold- Likes Yellow Hates Red
Brave- Likes Red hates Pink
Calm- Likes Green hates Red
Careful- Likes Green hates Blue
Docile- No Specific
Gentle- Likes Green hates Yellow
Hardy- No Specific
Hasty- Likes Pink hates Yellow
Impish- Likes Yellow hates Blue
Jolly- Likes Pink hates Blue
Lax- Likes Yellow hates Green
Lonely- Likes Red hates Yellow
Mild- Likes Blue hates Yellow
Modest- Likes Blue hates Red
Naive- Likes Likes Pink hates Green
Naughty- Likes Red hates Green
Quiet- Likes Blue hates Pink
Quirky- No Specific
Rash- Likes Blue hates Green
Relaxed- Likes Yellow hates Pink
Sassy- Likes Green hates Pink
Serious- No Specific
Timid- Likes Pink hates Yellow
As such, I've added a feature called Attitude. Attitudes are where Friendship comes in. If you do things with your pokemon that fit its "Attitude", your friendship with it goes up.
Attitudes by Nature
Affectionate - A nature in which the pokemon often shows great affection for its trainer. Often displayed in extreme ways such as a glomp or bite that may interfere with battle (in this case the opponent's attack won't count. Don't worry) - Naive, Quirky, and Sassy.
Argumentative - The pokemon will argue with other pokemon and occasionally the trainer, even over things that don't matter. - Rash, Sassy, Quirky.
Leader - Leads other pokemon when separated from trainers. When with trainer, is practically in sync with them. Adamant, Bold, Brave, Calm, Hasty, Quiet, Serious.
Hide - Usually a pokemon will hide from people or other pokemon and is easily firghtened out of doing things but occasionally rises to occasions. - Timid, Rash, Bashful, Lonely.
Mischievous - The Pokemon can cause total mayhem on purpose or wander off and cause trouble without meaning to. - Hasty, Naughty, Naive, Bold, Brave, Hardy
Sensative - A pokemon that's a bit sensative at times and tends to overreact. - Mild, Bashful, Timid, Sassy, Naive, Lonely
Shun - The pokemon would rather not battle and may try to avoid it but will battle if it has to. - Bashful, Lax, Quiet, Timid, Gentle, Relaxed
Caring - This Pokemon cares for other pokemon should they be injured or need help. - Careful, Docile, Gentle, Mild, Modest
Who Me? - Pokemon will pretend not to be as powerful as it's judge. - Modest, Bashful
All Givers - Pokemon that give their 100% in whatever their partnering human does. Adamant, Bold, Brave, Docile, Modest, Quirky, Serious
Linger - The Pokemon is often seen relaxing. - Calm, Gentle, Lax, Lonely, Mild, Quiet, Relaxed
Your Pokemon's Nature is randomly determined HOWEVER you get to pick up to two attitudes for the pokemon if it can have at least two. Some Natures only come with 1 Attitude, I believe.