At what point do our mental thought processes begin to differ, making some babies grow up into toddlers who can be content playing alone, and others needing constant attention? Maybe that is the "nature" part...our genetic disposition. An independent child might be praised for being so "good" and for doing things all by himself. This positive reinforcement could cause this child to grow into an adult with enough self-confidence to pursue any goals he sets. A very self-reliant person has a strong belief that he is perfectly capable of successfully dealing with the challenges life throws at him completely by himself, rather than taking guidance from other people or things, and this may make it more difficult to develop deep, intimate relationships. Furthermore, he might resent a person who acts "needy" of others because he is perfectly content being alone doing his own thing.
What about the child who constantly needs attention from those around him/her? If that attention is not received, would this child act out and misbehave in order to get it? This can cause a cycle of negative reinforcement in which the child grows up to be an adult who not only does inappropriate things to get attention, but is usually rewarded by getting what he wants from those around him who try desperately to avoid bad situations from occurring again. Anyone who has their mood altered so drastically based on the reactions of the people around him is setting himself up for disappointment and is probably more likely to have trust issues, low self-esteem, and therefore a more negative outlook on life.
On the other hand, what if that child did receive the attention he/she wanted growing up, in a nurturing way that made him feel secure in his surroundings, knowing that there will always be someone there for him/her? Would he be more confident to broaden his horizons and explore the world knowing there will always be someone supporting his dreams and there if he needs them? This person would probably be more likely to trust people, develop close relationships, and have a more positive attitude in life.
We've all had events in our lives that have caused us to be scared, worried, disappointed or sad from time to time, but given the same situation, each person reacts differently. What would cause one person to get angry and cause harm to others, such as in spousal abuse, gang fights, or even terrorist attacks, and cause another person to get depressed and become suicidal? Then there are the rest of the people who might initially feel down but then try to find a way to look at the same situation as a blessing in disguise and use this opportunity to make a change for the better in his or her life?
I'm a firm believer of "law of attraction" and that we are all just energy on different wavelengths so if you surround yourself with positive people it will help to raise your energy level, just as surrounding yourself with negative people can suck the life out of you. There is plenty of literature and films on "positive mental attitude" available, some of my favorite authors being Napoleon Hill, Deepak Chopra and the late Dr. Wayne Dyer. Anything that increases your endorphin levels (your "happy hormones") will put you in a better mood so if all else fails, turn to comedy and have a good laugh!