As a general point, people who are willing and helpful are much preferred to those who are not so co-operative. For a manager, there's really no contest if it's a choice between trying to persuade a recalcitrant person to shoulder a task (with the strong possibility that if they are made to do it, they'll execute it poorly in order to avoid being asked again), and handing it to the person who has proved accommodating in the past.
So, on the face of it, your attitude is welcome to your manager, and your behaviour is in line with what is expected from high-performing people these days. But at the same time, there are dangers. You could find yourself overloaded with tasks that aren't really your responsibility and that will be difficult to execute well. These could stop you doing your own work to the standard expected. And often, the additional jobs you are assigned will be those that others don't want to tackle - because they are tedious or low-status within the organisation.
This makes your additional work less interesting and more stressful, and, over time, you may become exclusively associated with this kind of task - which may damage your chances of promotion to a more senior role involving more prestigious work.
So, it's worth curtailing your immediate instinct to say yes. Clearly, you don't want to turn suddenly from a friendly, accommodating person into a grumpy refusenik who shuns all appeals for help. Nor do you want to impose some sort of 'work to rule', where you undertake to do only what is specified in your job description. But there are tactics that you can use to ensure that the extra work you agree to take on is appropriate and puts you in a good light.
The first thing to do when asked to shoulder an additional burden is to evaluate the suitability of the task. You will need some data, and this means asking questions, the principal ones being how important and urgent the task is. The answers will help you judge the feasibility and desirability of taking it on. If the task conflicts with your other priorities, you are within your rights to say pleasantly: 'I'd like to help you out, but I'm so loaded up with work (and here you can mention a key project or person) that I won't be able to.' If they continue to press you to take on the assignment, they will at least have noted that you are doing them a favour.
If you take on extra work because you've decided that you have the time and skills to do a good job and that doing it will help your relationship with someone, you still need to ask questions to make sure you know exactly what is required. You don't want to waste time and effort slaving away at some ill-conceived project, only to be criticised down the line when unexpressed expectations have not been met.
Sometimes you'll need to negotiate on three variables: time, cost and quality. There's a theory that you can only have two of these variables at any one time. You can have a job done to a good standard and cheaply, as long as you have lots of time to do it; you can have it quickly and cheaply, as long as the quality isn't crucial, and so on.
If you find the idea of changing your behaviour at work difficult, practise saying no at home. I'm willing to bet you show the same tendency to take on other people's problems and workloads there, too. It will do your self-esteem good to realise that you have your own priorities and have every right to pursue these in preference to those of other people.
You may find saying no even tougher at home, when loved ones seem to have legitimate calls on your time and attention. You can still be helpful to them while attending to your own needs. As they say on the airplane: 'Fit your own mask first before seeking to help others.'
Miranda Kennett is an independent coach. If you have an issue you'd like her to cover, e-mail: email@example.com.