Should I sound needy in my Statement of Purpose?
Short answer: No.
Long answer: This is from the perspective of a hiring PI. These days I am flooded with applications of candidates that seem very motivated, but give me little confidence that they are the right person for the job. This is mostly for two reasons: 1. lack of relevant background for the position; 2. poor self-presentation (for example, not reading the job description thoroughly). To address this, you need to have the right background (if necessary, you might take some additional courses), and make a convincing statement why you are the right person for the job.