Much of this multi-tasking can be contributed to technology; the cell phones, blackberries, tablets, laptops, and constant access to non-face to face communication means that we often have to come up with reasons why we can't do something, or have to utter that dreaded word "No," whose cousins "I can't" and "I'm not available at that time" tend to leave just as bitter a taste on our tongues. Perhaps we think that if they could peak through our window at that very moment, they would see that we were up to our necks in other obligations, but they can't, we don't want to appear lazy or unaccommodating, so we drop everything to do for another what they could often do for themselves.
What?!?! Do it themselves?!?! But they need me, they wouldn't do it correctly, and/or no one else could do it as well as me.
But won't they dislike me if I say no, or that I'm not available to do it?
Point #2. If that loved one who wants you to help them move apartments for the 6th time, or who needs to borrow your car again, or who needs a dog sitter while they go to the lake is going to stop loving and caring about you if you say "No, I can't absorb your obligation," that's not the kind of love you wanted in the first place. Love in its highest form is unconditional, and having people that will only be kind to you if you pick up their slack and stop what you're doing to help them with crisis #4,873, is not really the kind of person you want around anyway. If these types stop coming around because you aren't their butler/trust fund/catch-all drawer, you might be better off.
*Bonus* Again, this will lovingly give the favor-asker the chance to re-evaluate their choices. If they are making decisions that don't fit into their lives and is making them have to constantly rely on others to manage it, maybe having to miss out or do without will be inspiration enough to make some changes.
There are incredible charities that really do need people with the giving, generous spirit that you have. Find one that you're passionate about to share your kindness and let it serve as a reminder to yourself that you are a good person, but also not one who will get in the way of a learning moment for another. If you see that helping another will really not take away from your life, happiness or well-being, for example watching that niece that you adore anyway, or helping prune a friend's garden that results in some exercise and delicious home-grown tomatoes, then by all means, chip in!
But if it doesn't, I give you my blessing and encouragement to say gracefully, politely, but firmly, "No."