The "I-Am" breathing exercise is one way to open the heart. Breath in while saying "I" on the in-breath - focus your attention on your heart chakra at your sternum. Breath-out on the "Am". And repeat.

There is a danger of becoming overwhelmed - yes compassion and love are good emotions. To be always open may lead to burn-out. The overwhelm is about feeling all the suffering the world has to offer. And this could be too much at times. So the intellect comes in and draws boundaries. Take care that the boundaries don't become walls.

Be neutral - then I can shift to heart open when needed and close down when its too much.

Hope this helps.