From personal experience, I feel that not all of us have the tendency to say yes when we want to say no. The more assertive ones among us will say no when they want to say no. The same can be said of the individualists. This is not selfishness, but merely the fact that these individuals understand and treasure the concept of personal space. They understand that if they do not have time for themselves to recharge, they will surely not have quality time with others (since they will be so busy attending to other's needs).
However, does this mean that for the rest of us, we are foolish to place others before self? Actually, no. Personally, I feel that there's no right or wrong in this matter. It's merely a matter of personal preferences. For those of us who tend to say yes when we want to say no, we probably treasure friendship (where we try to appear agreeable) and/or altruism (when we attempt to help others) more than our personal time. While the benefit of having personal space is to enable us to recharge our energy, the benefit of helping others gives us a sense of fulfillment and to a certain degree, it gives us a sense of happiness as we know that we have made someone's day.
However, when we say yes to others, we need to self-reflect and question our own sincerity. Are we saying yes to others truthfully from the bottom of our hearts? Or are we merely trying to appear agreeable or helpful where what we really wish to achieve is to acquire a positive image of ourselves? When our answer is the former, then there is absolutely nothing wrong in us saying yes. However, it is important to note at this point that saying yes too often may reduce or even remove our personal time with ourselves that we are able to spend with our loved ones. Now, if our response is the latter, then we should really consider carefully if we really want to say yes, since doing do appears to be insincere.
There are certain situations in life where we have a higher tendency to say yes than others. Some examples include the initial stage of courtship (where we have an intention to win the affection of someone of our fancy through a demonstration of agreeableness and helpfulness), our interactions with our bosses (where saying yes implies a "can do" attitude that often leads to career advancement) and our intention to strengthen the bonds of friendships with our closed friends. As mentioned, it is important that we question the sincerity of our actions. If sincerity is lacking, our act of saying yes may seem to be manipulative. When this is the case, we should stop saying yes.
To conclude, I will say that we have a tendency to say yes in the face of love, authority figures or altruism. If sincerity exists in our gestures, all is well. In the absence of sincerity, it is best that we refrain from saying yes as it may seem manipulative. This is especially so if what we really want to say is no.