Whether you are happy employed, "between jobs," or suffering from habitual unemployment, it's helpful to be conscious of the image you are projecting during a job search. There is real potential damage to job seekers who appear desperate, with two rather simple explanations.
Employers may be more likely to take advantage. Applicants that appear the most desperate for work may be offered a lower salary and fewer responsibilities than their experience warrants. This can have short and long-term impacts on a career.
Desperate candidates may appear to be selling damaged goods. Even stellar qualifications can be tainted by desperate acts. Overly-aggressive tactics may give the employer the impression that something is wrong with you, and that makes your candidacy seem inherently flawed without even considering further information.
Consider salespeople. Better yet, consider recruiters!
What if a recruiter called you about a job and rambled on for an hour without taking a breath, saying the hiring company is the hottest around and are a much better employer than Facebook, Google, Amazon, Microsoft, etc.? Then they emailed you when the call ended to reiterate the same information. You might feel this recruiter is pushing pretty hard, and it might make you a bit skeptical about the job. Suppose the job at this company isn't a great fit for your background, yet the recruiter insists that you should interview anyway. Suspicious?
Try to picture this from the other side. Applicants should state their qualifications, but should also use some discretion to prevent the appearance of overselling.
What NOT to Do
Overly-aggressive follow-up - Recently I received a phone call from a job seeker who wanted to discuss a position he had recently applied for through my website. I didn't recognize the name, and when I looked in my inbox I found his application timestamped five minutes earlier. Immediate follow-up will almost always give the impression of desperation at worst and a lack of social skills at best.
The video above is from the comedy "Swingers" where the main character Mikey calls a woman he had met at a bar earlier in the evening and ends up leaving several awkward voicemails. Later in the movie Mikey and his friends debate the optimum wait time between meeting someone new and contacting them to ask them on a date.
Pandering - While it's often recommended to tweak or customize a resume to address specifics of a job requirement, there are limits to how customized a resume should be. As an example, listing the hiring company name and job title in a resume's objective is a clear example of overdoing it. (Don't write OBJECTIVE: Seeking job as 'JOBTITLE' working for 'COMPANY'.) Cutting and pasting elements of the job description into your resume is also frowned upon. Cover letters can (and should) include the hiring company name. Putting it on the resume is a mistake.
Underconfidence - Even highly skilled, qualified candidates can sometimes underestimate or understate their own skills and marketability. This is a common stereotype of technical candidates and there is a clear sector of the industry that is deeply uncomfortable talking about accomplishments.
Open to any job - Applicants who apply for positions that are either not a match for their skills or well below their career level will be looked upon as potentially flawed. Unemployed job seekers are often too quick to abandon their quest for the ideal role, and many voice their willingness to accept virtually any job offered.
Pleading for response/action - A polite request for a response to a job application or for an interview will be appreciated, but anything resembling begging will bring the applicant's qualifications into question. Competitive candidates under the current favorable market conditions (it's a seller's market) don't need to ask for a response.
Even if confidence doesn't come naturally to you, you need to recognize during job searches where genuine interest by a qualified candidate may be mistaken for desperation, and to balance a general interest in finding work with maintaining your dignity. There are clearly times when a job seeker is at a competitive disadvantage, but displaying confidence in your marketable skills (and honesty about weaknesses) is helpful to both get better results from applications and improve your position in any negotiations.